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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.