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Medical Dictionary of Health Terms: D-I
Medical Dictionary of Health Terms: D-I
D2 receptor: A type of dopamine receptor (see neurotransmitter receptors) that seems to be particularly important in addiction.
daidzein: A substance found in soybeans.
daily value: A guide to the amount of nutrients in a given food; Daily values are given in percentages based on a 2,000-calorie-per-day diet.
dandruff: A mild and common condition that is characterized by an itchy, flaky scalp and that may extend to the ears, face, and chest. Also known as seborrheic dermatitis.
de Quervain’s tendonitis: Painful swelling of the tendons at the wrist that move the thumb.
debility: Weakness or a loss of physical strength.
decibel: A unit of measurement for the loudness of a sound. The highest decibels indicate the loudest sounds.
declarative memory: Memory for facts or events (episodic memory); also called explicit memory.
decoction: An herbal product or tea made by boiling a plant in water.
decongestant: Type of medication used to relieve nasal congestion.
deep sleep: Stage of sleep where the brain is less responsive to outside stimuli.
deep venous thrombosis: A dangerous condition in which blood clots form in veins deep in the body, usually the legs. They may break off and block blood flow in the lungs, seriously damaging organs or causing death.
defenses: Coping strategies a person adopts to make it easier to operate in the world.
defibrillation: The delivery of an electric shock to the heart to stop an abnormal rhythm and restore a normal heartbeat.
defibrillator: A device that delivers an electric shock to the heart to restore normal rhythm. Used to treat cardiac arrest and other dangerous heart rhythm problems.
degenerative disease?: Any disease in which the organs or tissues in the body are damaged progressively over time.
degenerative disk disease: Normal, and sometimes painful, deteriorations in the disks of the spine that occur with age.
degenerative joint disease: Arthritis that occurs when the cartilage in joints breaks down over time; also called osteoarthritis.
degenerative spondylolisthesis: Arthritis of the spine that worsens over time; often caused by aging.
delayed sleep phase syndrome: A pattern of falling asleep and waking up later than wanted that tends to worsen progressively over time.
delirium: Sudden, severe confusion that occurs because of a mental or physical illness.
delta waves: Slow brain activity that occurs when a person is in deep sleep.
delusion: A false or irrational belief held by a person despite evidence to the contrary.
dementia: A loss of brain function that worsens over time and affects memory, thinking, behavior, and language.
dementia pugilistica: Loss of brain function, common among former boxers, caused by repeated blows to the head.
demineralization: The process by which bacteria destroy tooth enamel.
demulcent: A substance that soothes irritated tissues and mucous membranes.
dendrites: The parts of a nerve cell that receive signals from other nerve cells.
dendritic cells: Spidery-looking immune system cells that help protect the body from harmful substances.
denial: A defense mechanism characterized by the inability to recognize or admit that addiction is the cause of problems, rather than a solution or mere byproduct. Denial can also refer to the refusal to accept an upsetting reality, such as a serious illness or death or the feelings that follow either.
dental implant: A metal post inserted into the alveolar bone to support an artificial tooth or other prosthesis.
dentin: The layer of hardened tooth tissue under the enamel and around the pulp.
deoxyribonucleic acid: The substance found in the nucleus of cells that contains the genetic instructions for that living organism.
dermal papilla: A structure situated at the base of the hair follicle that contains nerves and blood vessels that fuel the cellular processes of the developing hair shaft.
dermatomyositis: A rare disease in which the muscles become weak and stiff and a skin rash appears.
dermis: The middle layer of skin that contains most of the skin’s structures, like collagen, nerves, glands, and hair follicles.
desiccated thyroid: An extract made of dried animal thyroid glands.
detoxification: The process of removing harmful, or toxic, substances from a person’s body.
detrusor instability: The sudden, strong need to urinate due to spasms in bladder muscles. Also called urge incontinence.
detrusor muscle: The layer of muscle in the bladder wall that squeezes urine out of the bladder.
detumescence: The softening of an erection.
diabetes: A disease in which the body does not properly produce or use insulin, resulting in abnormally high levels of sugar (glucose) in the blood.
diabetic ketoacidosis: A complication of diabetes in which substances called ketones build up in the blood to dangerously high levels.
diabetic retinopathy: A complication of diabetes that occurs when the small blood vessels in the retina are damaged. It can impair vision or even lead to blindness.
diaphragm: The dome-shaped sheet of muscle at the base of the lungs that helps move air in and out of the lungs.
diastole: The relaxation phase of the normal heart cycle.
diastolic blood pressure: The bottom number of a blood pressure reading, such as 134/78. It represents the pressure in the arteries when the heart relaxes between beats.
diastolic heart failure: The inability of the heart to relax properly between beats (diastole), making it difficult for the ventricles to fill completely with blood from the atria. This can occur when the heart muscle bulks up due to overwork or other causes or when the heart muscle stiffens and loses it flexibility.
diathermy: Use of high-frequency electric currents to heat deep muscle and joint tissue as a form of physical therapy.
dietary fiber: The part of plant foods the body can’t digest or absorb. Also called roughage.
dietary reference intakes: The recommended amount of vitamins and minerals individuals should consume per day.
dietary supplements: Vitamins, minerals, herbs, and other substances taken in hopes of improving health.
digestive tract: The series of hollow organs, including the mouth, esophagus, stomach, small intestine, large intestine (colon), rectum, and anus, which break down and digest food and expel waste.
digit: A finger or toe.
digital rectal examination: An exam in which a doctor inserts a lubricated, gloved finger into the rectum to check for abnormalities.
digitalis: A drug that increases the strength of heart muscle contractions.
dihydrotestosterone: A form of the hormone testosterone that spurs the prostate gland to enlarge (benign prostatic hyperplasia).
dilate: To widen or enlarge.
direct transmission: The immediate transfer of an infectious agent from a reservoir to a susceptible host by direct contact or droplet spread.
discoid lupus erythematosus: A rare form of lupus that causes a rash or scarring of skin.
disk: One of the small, shock-absorbing cushions found between the bones that make up the spinal column (vertebrae).
diskectomy: The surgical removal of part of a disk (a small, shock absorbing cushion located between the bones of the spinal column).
diskitis: Swelling of one or more of the pillow-like disks located between the bones of the spinal column.
dislocation: The movement of a bone from its normal position.
dithiolethione: A substance found in cruciferous vegetables, like cabbage, Brussels sprouts, and broccoli, which may play a role in cancer prevention.
diuretic: A drug that eases the heart's workload and decreases the buildup of fluid in the lungs and other parts of the body by promoting the excretion of water and salts. Diuretics (also called water pills) are used to treat high blood pressure, heart failure, and some congenital heart defects.
diverticulitis: The bulging out of small pouches or sacs of tissue from the colon wall.
diverticulum: A small pouch or sac of tissue that bulges out of the colon wall. The plural form is diverticula.
d-limonene: A substance found in citrus fruits.
DNA: An abbreviation for deoxyribonucleic acid, the substance in cells that contains the genetic instructions that direct their function.
DNR: Commonly used abbreviation for do-not-resuscitate order, a legal document that tells health professionals not to revive a person if his or her heart or breathing stops.
dominant: When the genetic trait carried by one allele of a gene pair eclipses the trait carried by another; for example, when a gene carries alleles for two different eye colors, the dominant eye color gene determines the person’s eye color.do-not-resuscitate order: A legal document that tells health professionals not to revive a person if his or her heart or breathing stops. Commonly referred to as a DNR.
dopamine: A chemical messenger (neurotransmitter) that affects movement and thought processes.
Doppler ultrasound: A test that uses sound waves to measure how fast blood is flowing through blood vessels.
dorsal: Near the back of the body or an organ.
dorsal kyphosis: A rounded or curved back caused by spinal fractures from osteoporosis. Commonly called dowager’s hump.
double blind: A medical study in which the researchers and participants don’t know which group is receiving the medication or treatment being studied and which is getting a placebo (fake, inactive version of the medication).
double-contrast barium enema: An x-ray test done to check for colon cancer or other bowel diseases.
dowager's hump: A rounded or curved back (resembling a hump) caused by spinal fractures from osteoporosis.
doxazosin: Medication used to treat high blood pressure or an enlarged prostate gland.
Dressler's syndrome: Inflammation of the sac surrounding the heart (pericardium).
DRI: Abbreviation for dietary reference intake, the recommended amount of vitamins and minerals individuals should consume per day.
drusen: Tiny yellow deposits in the retina of the eye.
dry eye: Stinging, burning, or irritation that occurs when the eye doesn’t produce enough moisture.
dry-powder inhaler: A small device that helps a person breathe in dry medication so it reaches the lungs.
dual energy x-ray absorptiometry: An x-ray test used to measure bone density and check for osteoporosis.
dual-photon absorptiometry: A test to measure bone density, usually in the spine or the hip.
duct: A tube or vessel in the body which carries the secretion of a gland; Secretion examples are tears, breast milk, etc.
duodenitis: Inflammation of the duodenum, which is the upper part of the small intestine.
duodenum: The upper part of the small intestine.
duplex Doppler ultrasound scanning: A tool that uses sound waves to reveal blood flow problems.
Dupuytren’s disease: A condition that deforms the hand, causing fingers to curl toward the palm.
dura mater: A thin protective membrane covering the brain and spinal cord.
durable power of attorney: A legal document in which an individual appoints another person to make medical, financial, or other decisions when the individual becomes unable to make those decisions.
dysarthria: A speech disability caused by an injury to the brain centers controlling the face, mouth, neck, or throat. People with dysarthria may be able to understand speech and form the right words in their mind but cannot articulate them.
dyspareunia: Painful sexual intercourse.
dyspepsia: Pain or discomfort in the upper abdomen; upset stomach or indigestion.
dysphagia: Difficulty chewing and swallowing food. Dysphagia is extremely common after a stroke.
dysplasia: Abnormal changes in cells of a tissue. The cells aren’t cancerous, but they can sometimes progress to cancer.
dyspnea: Difficulty breathing or shortness of breath.
dystonia: A disorder in which muscles twitch, causing uncontrollable twisting movements.
ear canal: A tube leading from the eardrum to the outer ear.
eardrum: A thin membrane separating the ear canal and middle ear.
earwax: A substance that lubricates the inner ear and helps protect it from dirt, damage, and infections.
EBCT: Abbreviation for electron-beam computed tomography, a high-speed imaging technology use to evaluate the heart and measure calcium deposits in arteries.
eccentric action: When muscles move joints by lengthening. Also known as cerumen.
ECG: An abbreviation for electrocardiogram, a test that measures the electrical activity of the heart and detects heart problems.
echocardiography: A diagnostic tool that uses high-frequency sound waves (ultrasound) to make images of the heart's size, structure and motion.
eclampsia: A serious condition related to high blood pressure that can threaten the life of a pregnant woman and her fetus.
ectopic pregnancy: Pregnancy in which a fertilized egg implants in an abnormal location outside the uterus, usually in the fallopian tubes. Ending the pregnancy is necessary.
ectropion: When an eyelid, usually the lower one, flips outward so that the inner surface is exposed.
eczema: A condition in which areas of the skin are dry, itchy, red, and cracked. Also known as atopic dermatitis.
ED: Commonly used abbreviation for erectile dysfunction, the inability to get or maintain an erection sufficient for intercourse.
edema: Swelling caused by abnormal accumulation of fluid in tissues.
EEG: Abbreviation for electroencephalogram, a test that measures the electrical activity of the brain and detects problems.
effusion: An abnormal buildup of fluid in a joint or tissue.
eighth cranial nerve: A nerve responsible transmitting sound and information about balance to the brain. Also called the auditory nerve.
ejaculation: A sudden discharge of a fluid from a duct; often used to describe the expulsion of seminal fluid from the urethra of the penis during orgasm.
ejection fraction: The percent of blood pumped out of the left ventricle with each heartbeat. A normal ejection fraction is in the range of 55% to 70%.
EKG: An abbreviation for electrocardiogram, a test that measures the electrical activity of the heart and detects heart problems.
elastin: A flexible, stretchy protein found in skin and connective tissue.
electrocardiogram: A test that measures the electrical activity of the heart and detects heart problems.
electroencephalogram: A test that measures the electrical activity of the brain and detects problems.
electrolysis: A permanent hair removal technique that destroys follicles one at a time with a hair-thin needle inserted into the base of the follicle.
electrolyte: Minerals in the body that are electrically charged and play an important role in body processes, such as regulating fluid levels in the body. Examples include calcium and sodium.
electromyography: A test that checks the health of muscles and the nerves that control them.
electron-beam computed tomography: A high-speed imaging technology use to evaluate the heart and measure calcium deposits in arteries. Sometimes referred to as EBCT.
electrophysiologic testing: A procedure used to provoke known or suspected arrhythmias.
elimination diets: A way of diagnosing food allergies in which suspected foods are removed from the diet one at a time until the food causing a problem is found.
ellagic acid: A chemical found in certain plants, such as raspberries and strawberries, that might help protect against cancer.
embolic stroke: A type of stroke that occurs when a blood clot that has formed elsewhere in the body breaks off and travels through the bloodstream until it blocks an artery that normally supplies blood to the brain.
embolism: Blockage of a blood vessel by a clot (an embolus) that has traveled from another part of the body.
embolus: A blood clot or particle that forms in one part of the body then moves through the bloodstream and lodges in a blood vessel elsewhere, blocking blood flow.
emetic: Any drug or other substance used to cause vomiting.
EMG: Abbreviation for electromyography—a test that checks the health of muscles and the nerves that control them.
emission: The discharge or release of a substance, usually a fluid.
emmenagogue: Herbs that stimulate menstrual blood flow.
enamel: The hard outside layer of tooth material.
encephalitis: A severe and sometimes deadly inflammation of the brain that can be caused by a number of different viruses.
encoding: A multistage process by which sensation, perception, or thought is transformed into neural representations that can be stored in memory.
endarterectomy: Surgical removal of plaque or blood clots in an artery.
endemic: Continually present among people in a geographic region.
endocarditis: An inflammation of the heart lining or valves, usually caused by bacterial infection.
endocardium: The inner layer of the wall of the heart.
endogenous opioids: Painkilling substances made by the body.
endometrium: The lining of the uterus.
endorphins: Substances in the body that reduce pain and create a feeling of well-being.
endoscope: A thin, flexible tube equipped with a light and camera that is used to see inside an organ or body cavity.
endoscopy: Inserting a flexible tube equipped with a light and camera into the body to see inside a body cavity or organ.
endothelins: Proteins that cause blood vessels to narrow and blood pressure to rise.
endothelium-derived relaxing factor: Chemicals in the body that cause blood vessels to expand or relax, lowering blood pressure. Often referred to as EDRF.
end-stage renal disease: Complete, or nearly complete, kidney failure. Dialysis or a transplant is needed for survival.
enkephalin: A chemical produced in the brain that reduces pain.
enteric nervous system: Part of the nervous system that controls the gastrointestinal system.
enteropathic: Disease affecting the intestinal tract.
enthesis: A place where a ligament, tendon, or muscle attaches to bone.
entropion: An eyelid, usually the lower lid, which folds inward so that the eyelashes rub against and irritate the surface of the eye.
enzyme: A substance that speeds up another chemical reaction. For example, digestive enzymes help speed up the digestion of food.
eosinophils: White blood cells that play an important role in allergic reactions.
epicardium: The outer layer of the wall of the heart.
epicondylitis: Pain and swelling in the tendons in the elbow, usually because of overuse.
epidemic: The occurrence of more cases of disease than expected within a population in a geographic area over a set period of time.
epidemiological study: An investigation of the links between certain behaviors or risk factors and the occurrence of disease or good health in a population.
epidermis: The outermost layer of skin.
epidural space: The space between the spinal cord and the bones of the spinal column where painkillers are injected.
epinephrine: A chemical that narrows blood vessels, increases heart rate, and helps trigger the fight-or-flight response to danger. Also called adrenaline.
EpiPen: A device used to inject a dose of medication (epinephrine) when a severe allergic reaction occurs.
epithelial cells: Cells which line organs and structures in the body, protecting or enclosing them.
epithelium: A layer of cells which lines organs and structures in the body, protecting or enclosing them.
erectile dysfunction: The inability to get or maintain an erection sufficient for intercourse. Sometimes referred to as ED.
erector spinae: A group of muscles and tendons in the back.
ergonomics: Designing and arranging work objects so that the user is comfortable, efficient, and less likely to be injured.
ergots: Substances derived from or made from a fungus; often used to treat headache.
eructation: The act of bringing up air from the stomach through the mouth with a characteristic sound. Commonly known as belching.
erythema: Redness of the skin because of widening of capillaries just below the surface of the skin.
erythema nodosum: Painful, red lumps beneath the skin; associated with Crohn's disease.
erythrocyte sedimentation rate: A test involving red blood cells; used to check for different infections, inflammations, and cancers.
erythropoietin: A hormone that controls red blood cell production.
esophageal manometry: A test to measure the pressure inside the lower part of the esophagus.
esophagitis: Irritation and swelling of the esophagus.
esophagus: The tube that carries food and liquids from the mouth to the stomach.
essential fats: Two fatty acids, omega-3 and omega-6, that the body needs for good health but can’t make so they must come from foods and supplements.
essential hypertension: High blood pressure with no known cause; also called primary hypertension.
esterified estrogens: Artificially made hormones used to manage menopausal symptoms.
esthetician: A person who specializes in non-medical skin care and beauty treatments.
estradiol: The primary form of the sex hormone estrogen produced by women.
estrogen: The main sex hormone in women.
estrogen receptor: A site on the surface of some cells to which estrogen molecules attach.
estrogen-replacement therapy: Use of medications containing the sex hormone estrogen by women to replace naturally-occurring estrogen lost during menopause.
etidronate: A medication used to treat bone loss due to Paget's disease.
eustachian tube: A tube connecting the middle ear and the back of the nose that lets air into the middle ear.
euthyroid: Having a thyroid gland that works properly.
excitatory neurotransmitter: A chemical that forwards a message from one neuron to another.
excitotoxin: A brain chemical that damages neurons.
executive functions: The component of thinking that organizes, plans, decides, and inhibits inappropriate impulses.
exercise: A structured program of physical activity that helps an individual become physically fit.
exercise stress test: The use of a treadmill, stationary bicycle, or other exercise machine while hooked up to heart-monitoring equipment. The test is used to determine if the heart's blood supply is sufficient and if the rhythm remains normal when the heart is stressed.
exophthalmos: A protrusion or bulging of the eye that occurs with Graves’ eye disease. Tissues behind the eye swell, forcing the eyeball forward. Also called proptosis.
exostosis: Abnormal bony growths in the ear caused by swimming regularly in cold water. Sometimes called surfer's ear.
experience sampling: A research technique for learning about people’s activity patterns and psychological processes that involves paging them at random times to obtain brief reports.
expression hopping: A common phenomenon whereby people jump to a different expression of addiction. For example, people with heroin addiction might transition to alcohol addiction. Hopping is especially common during the recovery process.
expression of addiction: The specific way in which a person manifests addiction, for example, through the use of cocaine, or compulsive gambling.
extend: To straighten out a joint (for example, extending the arms overhead).
external otitis: An infection or irritation of the outer ear, ear canal, or both. Also called swimmer’s ear.
extracapsular cataract surgery: A surgical technique to remove a cataract from the eye.
extract: A product made from substances that are drawn out of a plant or herb.
extraocular muscles: Six paired muscles that direct the eyes’ circular, side-to-side, and up-and-down movements.
extrinsic factor: An outside factor that has an effect on a person’s environment or well-being.
facet joints: Paired joints that link a vertebra to its neighboring vertebrae. They allow the spine to move as a unit.
facet rhizotomy: Surgical destruction of certain nerves and nerve roots to relieve pain.
false negative: Test results that show that a disease or substance isn’t present, even though it is.
familial combined hyperlipidemia: An inherited disorder in which the liver overproduces very low-density lipoprotein, causing high levels of cholesterol or triglycerides, or both.
familial hypercholesterolemia: An inherited disorder in which the liver cannot properly remove low-density lipoprotein particles from the blood, causing a very high cholesterol level.
fasting lipid profile: A laboratory test to determine the relative levels of high-density lipoprotein, low-density lipoprotein, and total cholesterol in the blood. Also referred to as a lipoprotein analysis, full lipid profile, or cholesterol profile.
fasting plasma glucose test: A blood test that determines the amount of sugar (glucose) in the blood after an overnight fast of at least eight hours.
fast-twitch fiber: One of two main types of skeletal muscle fibers. Fast-twitch fibers are recruited most heavily when bursts of power are needed, as in sprinting. See also slow-twitch fiber.
fat: One of the three major nutrients, along with carbohydrates and proteins.
fatigue: A lack of energy. A decrease over time in the ability to perform a physical or mental task.
fatty acids: Components of fats that can be used for energy by cells.
fatty streak: The first stage of atherosclerosis, a build-up of fatty materials in the wall of a blood vessel.
febrile: Feverish; having a high body temperature.
fecal impaction: A mass of dry, hard stool that gets lodged in the rectum.
fecal occult blood test: A test that checks for colorectal cancer by detecting blood in the stool.
femur: Thigh bone.
ferritin: A protein that captures and stores iron.
fetal alcohol syndrome: A collection of birth defects resulting from exposure of the fetus to alcohol during pregnancy.
fiber: A substance found in plant foods that the body can’t digest.
fibrillation: Rapid and uncoordinated contractions of heart muscle fibers. When this occurs, the heart can't properly contract or pump blood.
fibrin: A stringy protein that is the primary component of a blood clot.
fibrinogen: A protein that helps stop bleeding by aiding in forming blood clots.
fibroadenoma: A tumor that is not cancer; usually found in the breast.
fibroblast: A cell that helps form the collagen and elastic fibers of connective tissue.
fibroid: A tumor that is not cancer, which is found in the uterus.
fibromyalgia: A condition causing pain and tender spots throughout the body.
fibrous plaque: A buildup of fat on the inside of a blood vessel that interferes with blood flow.
fibula: The calf bone.
filtering procedure: A procedure to treat glaucoma in which a surgeon creates a drainage hole in the eye to relieve pressure.
finasteride: A medication for treating benign prostatic hyperplasia (enlargement of the prostate gland).
fixed joint: An area where two bones meet that is fixed, or doesn’t move.
flaccid: Soft, not erect.
flare: Reappearance or worsening of symptoms.
flat feet: A condition in which the arch is flat all the time or flattens when bearing weight.
flatulence: Excess gas in the stomach or intestines that is expelled from the anus.
flatus: Gas expelled through the anus.
flavonoids: Chemicals found in fruits, vegetables, wine, and tea that may protect cells from damage and have health benefits.
flex: Bend a joint (for example, flexing the knees).
flexibility: A component of physical fitness that refers to the range of motion available at a joint.
flight-or-fight response: Changes that occur in the body, such as rapid breathing and heartbeat, when a person is confronted with a perceived physical or emotional threat. Also called the stress response.
floaters: Tiny spots or lines that appear in a person’s field of vision when the jelly-like fluid of the eye breaks down with age. They are usually harmless, but can signal serious eye problems.
flow: A term coined by positive psychology pioneer Mihaly Csikszentmihalyi to describe an effortless, active state of being during which one loses awareness of time, self, and distractions. Flow hinges on balancing the size of a challenge and the level of skill a person brings to it.
fluorescein angiography: A test that uses special dye and a camera to examine blood flow in the retina.
fluoride: A mineral that protects teeth from decay and cavities.
fluoroscopy: A test that gives moving images of the inside of the body; could be likened to an x-ray movie.
foam cells: Lipid-laden cells, named for their foamy appearance under the microscope. As foam cells build up on the inside of blood vessel walls, they form a plaque that can block blood flow.
focal neuropathy: Damage to a specific nerve, causing pain or numbness.
focus words: Peaceful, relaxing words or phrases used during stress relief exercises.
follicle: A group of cells that form a small sac.
follicle-stimulating hormone: A hormone released by the pituitary gland that stimulates the production of eggs in women and sperm in men.
follicular unit: A small bundle that includes one to four follicles, oil glands, a tiny muscle, and other tissue.
follicular unit transplantation: Transplantation of follicular units, which mimic natural hair growth.
folliculitis: An inflammation of the hair follicle.
food allergies: Sensitivities to certain foods that can cause symptoms ranging from the mild (like hives) to the life threatening (such as anaphylaxis).
forehead lift: Cosmetic surgery to minimize creases on the forehead and between the eyebrows, and to lift sagging eyebrows.
fovea: A small dimple in the middle of the retina that provides sharp central vision.
fracture: A break in a bone.
FRAX tool: An algorithm for estimating the probability of breaking a bone because of low bone mass over a period of 10 years.
free fragment: Part of a disk in the back that has broken off from the main portion of the disk.
free radical: An unstable molecule in the body that plays a role in aging and can damage tissue; antioxidants help prevent free radical damage.
frequency: The pitch of a sound; measured by the speed at which sound waves vibrate.
frequency range: How much amplification a hearing aid produces in both high and low frequencies.
frequency response: The amplification that a hearing aid produces across different sound frequencies.
frontal lobe: Part of the brain that plays a role in determining consequences and choosing behaviors.
frontotemporal lobar degeneration: A brain disorder that can cause dementia, aphasia, neurotic behavior, and gradual changes in personality and emotional control.
frozen shoulder: Inflammation of various tissues of the shoulder, along with growth of abnormal bands of tissue that cause the shoulder to become so stiff that movement is severely limited.
fructose: A simple sugar found in corn syrup, honey, and many sweet fruits.
FSH: Abbreviation for follicle-stimulating hormone, a hormone released by the pituitary gland that stimulates the production of eggs in women and sperm in men.
functional gastrointestinal disorders: Gut problems that aren’t caused by an infection or structural problem with the gastrointestinal system.
functional incontinence: Recognition of the need to urinate but being unable to get to the bathroom in time due to mental or physical problems, such as limited mobility.
fundoplication: Surgery that restructures the stomach to prevent acid reflux.
fundus photography: Imaging test that provides multi-dimensional pictures of the back of the eyeball.
fungus: Organisms and microorganisms, such as yeasts and molds, that can live as a parasite on plants and animals.
gain: A hearing aid’s power, measured in the number of decibels that it can add to sound.
gait cycle: The cycle that the feet and legs make when walking, beginning when one heel hits the ground and ending when the same heel hits the ground again.
galactogogue: A substance that promotes breast milk production.
gamma-aminobutyric acid: A chemical messenger in the brain that may help decrease anxiety and promote slow-wave sleep. Often referred to as GABA.
ganglion: An abnormal but harmless mass of tissue, usually nerve cells.
ganglion cyst: A harmless sac of fluid on top of a joint or tendon, usually on the wrist or back of the hand.
gangrene: Death of tissue in part of the body because blood has stopped flowing there.
gastric: Relating to the stomach.
gastritis: Inflammation of the lining of the stomach.
gastroesophageal reflux disease: A condition in which food and acid flow back into the esophagus from the stomach, causing heartburn.
gastrointestinal: Relating to all or some of the organs of the digestive tract, from the mouth to the anus.
gastrointestinal tract: The digestive tract.
gene: Structures on chromosomes that are passed from parent to child. The basic unit of material that passes traits from parent to child.
gene therapy: Correction of a genetic defect by replacing an abnormal gene with a normal gene.
generic drug: A copy of a brand-name drug whose patent has expired. These drugs are less expensive than brand-name drugs.
genetic: Referring to inherited characteristics or genes.
genistein: An antioxidant chemical found in certain plants, like soybeans.
GERD: Abbreviation for gastroesophageal reflux disease, a condition in which food and acid flow back into the esophagus from the stomach, causing heartburn.
geriatric care manager: A trained medical professional who helps families who are caring for older adults.
geriatrician: A physician who specializes in the care of older patients.
gestational diabetes mellitus: A form of diabetes that appears during pregnancy.
GFR: Abbreviation for glomerular filtration rate, the rate at which the kidneys filter excess water and other wastes. A test by the same name is used to determine how well the kidneys are functioning.
giant cell arteritis: Inflammation and damage to blood vessels that supply the head and neck.
gingiva: Another term for gums: a form of oral tissue that covers the roots of teeth and surrounding bone.
gingivitis: Inflammation of the gums.
gland: Any organ or tissue that secretes fluids, such as hormones, for use elsewhere in the body or as waste.
glans penis: The head of the penis.
glaucoma: A condition in which the pressure inside the eye is too high, causing eye damage.
Gleason score: In men with prostate cancer, the Gleason score provides a rough estimate of how fast the cancer is growing.
glenohumeral joint: A shoulder joint that connects the ball of the humerus to the glenoid.
glenoid: The socket of the scapula that connects to the humerus at the shoulder.
glomerular filtration rate: The rate at which the kidneys filter excess water and other wastes. A test by the same name is used to determine how well the kidneys are functioning.
glomeruli: Tiny clusters of capillaries in the kidneys that filter waste products from the blood.
glucagon: A hormone produced in the pancreas that raises blood sugar levels.
glucocorticoids: Steroid hormones released by the adrenal gland when there seems to be a threat, and the stress response is triggered.
glucose: A simple sugar that is the body’s main source of energy.
glucose intolerance: Blood sugar levels that are higher than normal, but not yet high enough for a diagnosis of diabetes.
glutamate: A chemical messenger in the brain that may play a role in mood disorders and schizophrenia.
glutamic acid decarboxylase: A protein found in beta cells, the cells that create insulin.
glycemic index: A ranking of foods according to how quickly and how much they raise blood sugar.
glycemic load: A ranking of how much a typical serving of a particular food will raise blood sugar.
glycogen: The body’s fuel reserves. This substance, stored primarily in the liver and muscles, is later converted into glucose to provide cells with energy.
glycosylated hemoglobin: The product formed by the attachment of glucose (blood sugar) to hemoglobin (the oxygen-carrying protein in red blood cells). Usually referred to as hemoglobin A1c, or HbA1c.
GnRH: Abbreviation for gonadotropin-releasing hormone, a hormone responsible for the release of follicle-stimulating hormone and luteinizing hormone from the pituitary gland.
GnRH antagonists: Drugs used to treat prostate cancer by blocking the release of luteinizing hormone that do not cause a temporary surge in testosterone.
goblet cells: Cells that produce mucus that line the gastrointestinal tract and lungs.
goiter: An enlarged thyroid gland that creates a lump in the neck.
gonadotropin-releasing hormone: A hormone responsible for the release of follicle-stimulating hormone and luteinizing hormone from the pituitary gland.
gonioscopy: A test used to detect glaucoma that examines the area where fluid drains out of the eye.
gout: A form of arthritis that occurs when uric acid builds up in the joints; The big toe, knee, and ankle joints are most often affected.
graft: Transplanting tissue from one part of the body to another.
gram: A metric unit of weight equivalent to one-thousandth of a kilogram.
Graves’ disease: An autoimmune disease that causes the thyroid gland to produce too much thyroid hormone. It is usually accompanied by an enlarged thyroid gland and swollen, bulging, red eyes that appear to stare, as well as occasional double vision and vision loss.
greenstick fracture: A fracture in a young, soft bone in which the bone bends and breaks only on the outer edge of the bend.
grief: A broad range of reactions to bereavement, including sadness, tears, shock, confusion, and anger, among others.
growth factor: A substance produced by the body to stimulate tissue growth.
G spot: The area of sexually sensitive tissue located on the roof of the vagina just inside the opening. Also known as the Grafenberg spot.
guardianship: A legal process of appointing a person to make decisions about important matters (such as health and finances) for someone who is unfit to make those decisions on his or her own.
gullet: The esophagus.
gum disease: Diseases including gingivitis and periodontitis that attack the gum tissue and the structures supporting the teeth. Also called periodontal disease.
gums: A form of oral tissue that covers the roots of teeth and surrounding bone. Also called the gingiva.
hair bulb: The root of the hair shaft.
hair cells: Cells in the ear that transmit sound messages to the brain and play a vital role in hearing.
hair shaft: The portion of hair that extends beyond the surface of the skin. It contains three layers: the cuticle, cortex, and medulla (not always present).
hallucination: A perception of something that is not really there.
hammertoe: A sometimes painful condition in which the toe curls up and under (resembling a hammer).
happiness: Feelings of contentment or joy; the overall experience of pleasure and meaning in life.
happiness set-point: An individual's baseline level of happiness, determined largely by genetics, around which moods fluctuate. After reacting to positive or negative life changes, people tend to return to their happiness set-points.
harm reduction therapy: A treatment strategy aimed at minimizing the harm associated with an object of addiction. This strategic approach helps people learn how to limit the degree to which they use their object of addiction, or limit the risks associated with their use, but they do not necessarily stop altogether.
Hashimoto's thyroiditis: A disease in which the body’s immune system prevents the thyroid gland from producing enough thyroid hormone.
hay fever: A commonly used term for allergic rhinitis—seasonal or year-round allergic condition marked by sneezing, runny nose, and congestion. The most common type of allergy, it is caused by an IgE-mediated immune response to inhaled airborne allergens.
HbA1c: Abbreviation for hemoglobin A1c, the product formed by the attachment of glucose (blood sugar) to hemoglobin (a protein in red blood cells). A test for HbA1c is a useful measure of blood sugar control.
HDL: Abbreviation for high-density lipoprotein, the so-called good cholesterol. This lipoprotein (a substance made up of fats and protein) is believed to remove cholesterol from the arteries.
health care proxy: A legal form allowing a person to appoint someone else to make their medical decisions. Also known as a durable power of attorney for health care.
hearing aid: An electronic device worn in or behind the ear by people with hearing problems that makes sounds louder.
heart attack: The common term for a myocardial infarction. It refers to the damage that occurs when blood flow to part of the heart is blocked or drastically restricted. The blockage usually stems from the rupture of an atherosclerotic plaque.
heart block: The difficulty or inability of the electrical signal that triggers a heart contraction to pass through the atrioventricular node.
heart failure: The inability of the heart to pump enough blood to meet the needs of the body's organs.
heart murmur: An extra or unusual sound heard during the heartbeat. A murmur may or may not be a sign of a problem in the heart.
heart rate: The number of times the heart contracts in a minute, normally 60–100 times.
heartburn: A burning pain in the chest or throat, caused when stomach acid backs up into the esophagus.
Heberden’s nodes: Bony bumps found on the finger joints in some people with osteoarthritis.
hedonic: Devoted to pleasure.
hedonic treadmill: The human tendency to adapt to new circumstances and eventually consider them to be normal, so the emotional effects (negative or positive) generated by a change fade over time.
heel spur: An abnormal growth of bone or calcium on the heel bone.
Helicobacter pylori: Bacteria that damages the lining the stomach. It is to blame for most ulcers and stomach inflammation. Often called H. pylori.
helper T cells: Cells that help fight disease by activating and directing other immune system cells.
hematemesis: The vomiting of bright red blood, indicating bleeding in the upper digestive tract.
hematoma: Blood that leaks out of blood vessels and collects in the body.
hematuria: Blood in the urine.
hemianopia: Poor vision or blindness in half of the visual field, affecting one or both eyes.
hemiparesis: Muscle weakness on one side of the body.
hemiplegia: Paralysis on one side of the body.
hemochromatosis: A genetic condition in which the body absorbs and stores too much iron.
hemodialysis: The use of a machine to remove wastes and impurities from the blood when the kidneys are not working properly.
hemoglobin: The oxygen-carrying substance that gives red blood cells their color.
hemoptysis: Coughing up or spitting up blood from the lungs.
hemorrhage: Bleeding from a damaged blood vessel.
hemorrhagic stroke: A type of stroke that occurs when a blood vessel leaks or bursts inside the brain, cutting off the supply of oxygen and nutrition to that part of the brain.
HEPA filter: Abbreviation for high-efficiency particulate air, a type of highly efficient air filter.
heparin: A drug that prevents blood from clotting.
herd immunity: Protection occurring when so many people in a region are immune to an infectious disease that it can’t spread to others.
herniated disk: When part of a spinal disk bulges out of a tear or weak spot in the disk’s tough outer shell. Also known as prolapsed disk.
herpes zoster: A painful blistering skin rash caused by the chicken pox virus; also known as shingles.
hertz: The measurement of a sound’s frequency.
hiatal hernia: When part of the stomach pushes upward into the chest through an abnormal opening in the diaphragm.
high blood pressure: When blood flowing through arteries pushes on artery walls with abnormally high force over a sustained period of time. Also called hypertension.
high-density lipoprotein: So-called good cholesterol. This lipoprotein (a substance made of fat and protein) removes cholesterol from arteries.
high-intensity focused ultrasound: A treatment that destroys tumors with heat generated by ultrasound energy.
hippocampus: Part of the brain that plays an important role in processing long-term memories.
hirsutism: Excessive facial or body hair in women.
histamine: A substance released by the immune system when it is exposed to an allergen. Histamines cause many allergy symptoms.
HIV: Abbreviation for human immunodeficiency virus, the virus that leads to acquired immunodeficiency syndrome (AIDS). HIV damages immune cells so that they are no longer able to fight off other infections.
hives: An itchy rash of usually short duration. Also known as urticaria.
HLA: Abbreviation for human leukocyte antigen, a protein found on the surface of white blood cells that helps the body recognize and fight foreign substances.
Holter monitor: A small machine worn for a day or more to continuously record the heart’s electrical activity. Holter monitors are used to help diagnose heart rhythm problems.
homeostasis: The body’s ability to keep blood pressure, temperature, water levels, oxygen levels, and more set at the right levels for cells to survive.
homocysteine: An amino acid formed as part of the normal breakdown of protein.
homocystinuria: A rare genetic disease that causes blood levels of homocysteine to rise too high.
hormone therapy: Use of medications to boost levels of hormones that decrease with age.
hormones: Powerful chemicals that affect many processes in the body, including sexual function, mood, and growth.
host: A person or other living organism that can be infected by a virus or other pathogen under natural conditions.
hot flash: A sudden, intense, hot feeling in the face or upper part of the body, along with rapid heartbeat, sweating, and flushing. A symptom of menopause.
HPA: Abbreviation for hypothalamic-pituitary-adrenal axis, a system that controls many hormonal activities in the body, including the stress response.
human immunodeficiency virus: Usually abbreviated as HIV, the virus that leads to acquired immunodeficiency syndrome (AIDS). HIV damages immune cells so that they are no longer able to fight off other infections.
human leukocyte antigen: A protein found on the surface of white blood cells that helps the body recognize and fight foreign substances.
humectant: An agent used in moisturizers; binds water to the skin to promote hydration.
humoral immunity: Immunity from infection and disease that comes from the release of antibodies in the blood.
Huntington's disease: An inherited disorder characterized by involuntary jerky movements and dementia.
hyaluronic acid: A hydrating sugar secreted by cells.
hydrogen breath test: A test used to diagnose gastrointestinal problems; it measures the amount of hydrogen in the breath, which rise when food isn’t being absorbed properly.
hydrogenation: The addition of hydrogen to a compound. Hydrogenation is used to solidify liquid vegetable oils. The process creates trans fats, which are harmful to the heart and blood vessels.
hydroxyapatite: A hard substance that is a major component of bones and teeth.
hygiene hypothesis: The theory that modern cleanliness means children are not exposed to as many germs at an early age, which results in a more allergies.
hyperaldosteronism: Overproduction of the hormone aldosterone, which causes the kidneys to retain sodium and water, often leading to high blood pressure.
hyperalgesia: An increased sensitivity to pain.
hypercholesterolemia: High levels of cholesterol in the blood.
hyperglycemia: An abnormally high amount of sugar in the blood.
hyperinsulinemia: High levels of insulin in the bloodstream.
hyperlipidemia: High levels of blood lipids (fats and waxes such as cholesterol).
hyperopia: Difficulty seeing objects that are nearby; farsightedness.
hyperparathyroidism: Overproduction by the thyroid glands of parathyroid hormone.
hyperplasia: Increased production of cells in a normal tissue or organ; may be harmless or a sign of precancerous changes.
hyperplastic polyp: Noncancerous growths commonly found in the colon and rectum.
hypertension: The medical term for high blood pressure. Hypertension is a major risk factor for stroke, especially hemorrhagic and lacunar strokes, because it puts excess stress on the walls of blood vessels and damages their delicate inner lining.
hypertensive cerebellar hemorrhage: A stroke in which there is bleeding in the cerebellum because high blood pressure has weakened arteries in the brain.
hypertensive intracerebral hemorrhage: A stroke in which high blood pressure causes a blood vessel deep in the brain to rupture.
hyperthyroidism: A condition in which the thyroid gland makes too much thyroid hormone.
hypertriglyceridemia: High levels of triglycerides in the blood. A normal triglyceride level is less than 150 mg/dL.
hyperuricemia: Abnormally high levels of uric acid in the blood.
hypnagogic hallucinations: Vivid dream-like sounds or images that occur while on the verge of falling asleep.
hypnogram: A diagram that summarizes the stages of sleep recorded in a sleep laboratory.
hypnotic: An agent that promotes and aids sleep.
hypoglycemia: A condition in which blood sugar drops to an abnormally low level.
hypogonadism: Extremely low levels of testosterone in circulation.
hypomania: A mild form of mania, in which a person has lots of energy, talks faster than normal, has racing thoughts, and elevated mood.
hypopnea: Breathing that is more shallow and slow than normal.
hypotension: The medical term for low blood pressure.
hypothalamic-pituitary-adrenal axis: A system that controls many hormonal activities in the body, including the stress response. Often called the HPA axis.
hypothalamus: A small area in the brain that produces hormones that control body temperature, hunger, moods, the stress response, and other key functions.
hypothyroidism: A disease in which the thyroid gland does not make enough thyroid hormone.
hypoxia: When all or part of the body doesn’t get enough oxygen.
hysterectomy: An operation to remove a woman’s uterus.
iatrogenic: Complications or poor effects caused by medical treatment.
IBD: Abbreviation for inflammatory bowel disease, a general term for two disorders—ulcerative colitis and Crohn's disease—that cause the intestines to become swollen and inflamed.
ICD: Abbreviation for implantable cardioverter defibrillator, a device implanted in the chest and connected to the heart that delivers a shock to stop a potentially deadly rhythm and restore a normal (sinus) rhythm.
ice pick headache: Stabbing, very intense headaches that come on suddenly and are very brief.
idiopathic: A condition or disease of unknown origin.
IgE: Abbreviation for immunoglobulin E, the substance responsible for most allergic reactions.
ileum: The final section of the small intestine.
iliopsoas muscles: Two muscles, running from the end of the spine to the thighbone, that are responsible for lifting the knee.
immediate hypersensitivity: A category of allergic reaction, triggered by specific allergens and involving IgE. The majority of allergic reactions to pollens, pets, dust, mold, food, and insect venom are of this type.
immobilize: To restrict the movement of a limb or other part of the body to help in healing.
immunity: The body’s ability to resist infection and disease.
immunization: Injection of harmless bacteria or viruses to spur the body to produce antibodies so it can resist a particular disease.
immunoglobulin: Substances made by the immune system that attack foreign substances. Also known as antibodies.
immunoglobulin E: The substance responsible for most allergic reactions.
immunologically privileged site: A part of the body—such as the eye or ovaries—where the immune system isn’t able to function because it may damage the tissue there.
immunosuppressant drug: Medication that stifles the body's immune response; often given after an organ transplant to prevent rejection of the new organ.
immunotherapy: Treating disease by enhancing or suppressing the body’s immune system.
impacted: Something firmly fixed into place, such as a wisdom tooth.
impaction: A mass of hardened feces blocking the rectum or colon.
impaired fasting glucose: Blood sugar levels that are higher than normal but not yet diabetic. This term is used when the high blood sugar levels are found with a fasting plasma glucose test.
impaired glucose tolerance: Blood sugar levels that are higher than normal but not yet diabetic. This term is used when the high blood sugar levels are found with an oral glucose tolerance test.
impedance hearing testing: A test that sends sound waves to the eardrum to determine if a problem in the middle ear is causing hearing loss.
implantable cardioverter defibrillator: A device implanted in the chest and connected to the heart that delivers a shock to stop a potentially deadly rhythm and restore a normal (sinus) rhythm.
impotence: The inability of the penis to become firm or to stay firm enough to have sexual intercourse.
in situ: Latin for in place.
inactivated vaccines: Vaccines containing microbes that have been killed, and, therefore, are unable to cause disease.
incision: A cut made into the skin or an organ during surgery.
incontinence: Involuntary passing of urine or feces.
incubation period: The time between when a person is exposed to an infection and when symptoms appear.
infarct: An area of dead tissue caused by insufficient blood supply.
infarction: The death of tissue due to a lack of blood.
infection: The growth of harmful organisms that can cause disease, such as bacteria, in the body.
infectious arthritis: Arthritis caused by harmful organisms such as bacteria.
inferior myocardial infarction: Heart attack involving the back part of the heart-muscle wall.
inferior vena cava: The large vein that carries blood from the lower half of the body to the heart.
infiltrating cancer: A cancer that has spread from where it first developed into surrounding tissue.
inflammation: The body’s reaction to injury or infection. It is characterized by swelling, heat, redness, and pain.
inflammatory bowel disease: A general term for two disorders, ulcerative colitis and Crohn's disease, that cause the intestines to become swollen and inflamed. Often referred to as IBD.
infusion: The slow injection of a fluid into a vein or tissues.
ingrown toenail: A condition in which the side of a toenail pierces the skin, causing pain, swelling, and sometimes infection.
inhibitory neurochemical: A chemical that stops the transmission of a message from one nerve cell to another.
injectable fillers: Substances injected into the skin to fill in wrinkles or add plumpness to the lips.
injection: Inserting fluids, such as medications, into the body by means of a hollow needle and syringe.
innate immunity: The body’s basic defenses against disease or infection that are present from birth.
inner ear: The deepest part of the ear, consisting of the cochlea and the labyrinth.
inoperable: A condition that cannot be treated by surgery.
insomnia: The inability to fall asleep or remain asleep long enough to feel rested.
insulin: A hormone made by the pancreas that controls the amount of glucose (sugar) in the blood.
insulin resistance: A condition in which the body produces insulin but can’t use it properly. This leads to diabetes.
insulin-dependent diabetes: Now called type 1 diabetes. It occurs when the body doesn’t produce enough insulin to control blood sugar levels properly.
intense pulsed light: A device that emits broadband light to improve the look of the skin or remove unwanted hair.
intensity: In exercise, a measure of how hard the body is working. Cues like breathing, talking, and sweating help measure intensity through perceived exertion.
interferons: Proteins made by the body to protect against viruses, bacteria, and other harmful agents.
interleukins: A group of substances that act as messengers in the immune system.
intermediate-density lipoprotein: A type of lipoprotein. It consists of remnants of very-low-density lipoprotein (VLDL) that eventually turn into low-density lipoprotein (LDL).
intermittent claudication: Pain and cramping in the legs during exercise that is caused by narrowed or blocked arteries. Also known as peripheral artery disease of the legs.
international unit: An internationally accepted amount of a substance based on its biological activity or effect; used as a measurement for fat-soluble vitamins.
interpersonal therapy: Short-term talk therapy focused on identifying and addressing problems in current relationships and building social skills.
intervention: A planned, often group, meeting with a person with addiction, with the aim of overcoming denial and inducing the individual to seek treatment.
intervention study: A study that compares one group that receives a medication or other therapy (an intervention) and another group does not (controls).
intervertebral disk: One of the small, shock-absorbing cushions located between the vertebrae of the spine.
intervertebral foramen: The opening between vertebrae through which a spinal nerve exits the spinal column (plural: foramina).
intolerance: An adverse reaction that may have similar symptoms to an allergic reaction but does not engage the immune system, and thus is not an allergy.
intracerebral hemorrhage: A hemorrhagic stroke caused by the rupture of a blood vessel and subsequent bleeding into the brain tissue.
intraocular lens: A small artificial lens permanently fixed inside the eye to replace the natural lens during cataract surgery.
intrinsic sphincter deficiency: Inability of the urinary sphincter to close completely.
iodides: Salt compounds containing iodine that are used to control severe hyperthyroidism in special circumstances. They work by decreasing the thyroid gland’s production and secretion of thyroid hormone.
iris: The colored ring in front of the lens that controls the size of the pupil and how much light enters the eye.
irritants: Substances such as tobacco, wood smoke, perfumes, and others that cause allergy-like symptoms, although the response is not an allergic reaction.
ischemia: Inadequate blood supply to an organ or part of the body.
ischemic heart disease: The most common form of heart disease, in which narrowed or blocked coronary arteries have difficulty supplying sections of the heart muscle with the blood they need (ischemia).
ischemic stroke: A stroke caused by an interruption in the flow of blood to the brain; almost always caused by a blood clot blocking a blood vessel.
islets of Langerhans: Clusters of hormone-producing cells, including alpha and beta cells, that appear throughout the pancreas.
isolated systolic hypertension: A form of high blood pressure (hypertension) characterized by elevated systolic blood pressure and normal diastolic pressure.
isometric: An action in which a muscle generates force but does not contract or extend enough to move a joint, such as when pushing against an immovable object.
isotonic: An actions in which a muscle generates force by contracting or lengthening to move an attached joint through its range of motion, such as lifting a dumbbell from knee-height to shoulder-height.
IU: Commonly used abbreviation for international unit, an internationally accepted amount of a substance based on its biological activity or effect; used as a measurement for fat-soluble vitamins.
As a service to our readers, Harvard Health Publishing provides access to our library of archived content. Please note the date of last review or update on all articles. No content on this site, regardless of date, should ever be used as a substitute for direct medical advice from your doctor or other qualified clinician.
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