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5-alpha reductase: A chemical that changes the sex hormone testosterone into a substance called dihydrotestosterone. This hormone can cause the prostate gland to grow abnormally.

abdominal muscles: A flat sheet of muscles on the front of the abdomen, between the ribcage and the pelvis.

abdominoplasty: A procedure to remove excess abdominal skin and tighten the underlying stomach muscles. Also known as a tummy tuck.

abduction: Movement of a body part, such as an arm or leg, away from the center of the body.

ablation: A form of treatment that uses electrical energy, heat, cold, alcohol, or other modalities to destroy a small section of damaged tissue.

abrasion: A scraping or rubbing away of the skin or other surface.

abscess: Pus that collects in a pocket of swollen, red tissue. Often occurs on the surface of the skin.

abutment: A tooth or implant to which a fixed prosthesis is anchored.

acceptance-based therapies: Psychotherapy techniques that use mindfulness to help a person recognize and accept thoughts and feelings but not be controlled by them.

accommodation: The eye's ability to focus on objects that are close.

ACE: Abbreviation for angiotensin-converting enzyme, an enzyme that converts the inactive form of the protein angiotensin (angiotensin I) to its active form—angiotensin II.

ACE inhibitor: Abbreviation of angiotensin-converting enzyme inhibitor, a drug used to treat high blood pressure and congestive heart failure.

acetabulum: A curved, bowl-shaped depression in the outer part of the hipbone. The ball-shaped portion at the top of the thighbone fits into this space to form the hip joint.

acetaldehyde: The main breakdown product of alcohol metabolism; accumulation of it in the bloodstream may produce flushing (a feeling of heat in the face or chest) and vomiting.

acetaminophen: A common, over-the-counter drug used to reduce fever and relieve mild to moderate pain, but which does not reduce redness or swelling (inflammation).

acetylcholine: A neurotransmitter (a chemical messenger that sends signals between brain cells) that plays roles in attention, learning, and memory.

Achilles' tendon: A band of connective tissue that connects the calf muscle to the heel bone. This tissue is prone to swelling and/or rupture.

achlorhydria: A condition in which the stomach produces little or no acid. This can affect digestion, cause stomach pain, and keep the body from absorbing vitamins and nutrients.

acne: An inflammatory disease resulting from excess sebum production, follicle plugging, and increased bacterial production.

acquired immunodeficiency syndrome: Usually abbreviated as AIDS. This is the most advanced stage of infection with the human immunodeficiency virus (HIV), which can severely weaken the immune system. People with AIDS get many infections, often from diseases that don't affect people with healthy immune systems.

acquisition: The term given to the brain's absorption of new information to begin creating a memory.

acromioclavicular joint: A shoulder joint that connects the clavicle to the scapula.

actin: One of the proteins that allows cells to move and muscles to contract.

actinic keratosis: Scaly pink or red-brown raised spots or patches on the skin caused by overexposure to the sun. Actinic keratosis may be a precursor to skin cancer.

active surveillance: A strategy for managing early prostate cancer in which a man has regular checkups but does not undergo treatment until the disease shows signs of worsening.

acupressure: Using the thumb or fingers to apply pressure to particular spots, or pressure points, on the body in order to relieve pain.

acupuncture: A treatment based on Chinese medicine. Thin needles are inserted into the skin at specific points on the body. This therapy is used to treat pain and various health problems and to reduce stress.

acute: A condition that comes on suddenly, often with severe, but short-lived symptoms.

acute pain: Severe pain that occurs suddenly and usually lasts a short while.

acute urinary retention: A sudden inability to empty the bladder. Causes include an enlarged prostate gland (in men) or bladder muscle problems.

adaptability: The ability of an organism to change genetically in a way that allows it to deal better with its environmental conditions.

adaptive immunity: The ability of the body to learn to fight specific infections after being exposed to the germs that cause them.

addiction: Loss of control over indulging in a substance or performing an action or behavior, and continued craving for it despite negative consequences.

adduction: Movement of a body part toward or across the midline.

adenocarcinoma: A type of cancer that grows in the layer of tissue known as the epithelium. This tissue lines organs and structures in the body, protecting or enclosing them.

adenoma: A benign growth found in the layer of cells that lines certain organs (epithelial cells).

adenosine triphosphate: An energy-storing molecule that is found in all human cells. Usually abbreviated as ATP.

adequate intake: An estimate of the amount of a nutrient needed by healthy people. The Adequate Intake is used when there isn't enough information to set a recommended dietary allowance (RDA).

adhesion: A band of scar-like tissue that forms between two surfaces inside the body, connecting tissues or organs which are not normally connected.

adipose tissue: Fat-filled tissue.

adjuvant therapy: Extra therapy given after a primary treatment, to increase the effectiveness of the primary treatment. For example, using chemotherapy after surgery or radiation treatment for cancer.

adrenal glands: Glands that sit on top of each kidney and secrete stress hormones.

adrenaline: Stress hormone that puts the body on high alert. Changes include faster heartbeat, more rapid breathing, greater energy, and higher blood pressure. Also called epinephrine.

adult day services: Centers providing daytime services to adults who need supervision, social support, or assistance with daily activities.

adulterant: An ingredient in a medicinal product (herb, supplement, or prescription drug), which dilutes the purity of the product and does not contribute to its therapeutic effects.

advance care directive (or advance medical directive): A legal document that describes the kind of medical care a person want if an accident or illness leaves him or her unable to make or communicate decisions.

advanced sleep phase syndrome: A pattern of falling asleep and waking up earlier than wanted that worsens progressively over time.

aerobic: Any process that requires oxygen. Often used to describe a form of exercise, aerobic exercise.

aerobic exercise: Physical activity that speeds breathing, improves heart and lung function, and offers many other health benefits. Examples include brisk walking, running, or cycling.

aerophagia: Excessive swallowing of air.

aesthetician: Licensed skin care professional who performs procedures such as deep cleansing, low-grade chemical peels, microdermabrasion, and postsurgical skin care.

after-cataract: Clouding of the lens of the eye that can occur months or years after cataract surgery.

age-related cognitive decline: The slight loss of memory and slowing of the brain's information processing that occurs with normal aging.

age-related macular degeneration: A potentially blinding condition that destroys sharp central vision.

agnosia: A rare disease in which a person can't recognize objects, shapes, or people. Often due to a brain or neurological condition.

agonist: 1) A substance that triggers a physiological response when it combines with a receptor. 2) A muscle whose contraction is opposed by another muscle.

agoraphobia: Fear and avoidance of public places and open spaces.

AIDS: abbreviation for acquired immunodeficiency syndrome, the most advanced stage of infection with the human immunodeficiency virus (HIV).

AIDS dementia: A loss of brain function that occurs rapidly in some AIDS patients; marked by forgetfulness, difficulty thinking, and trouble focusing.

albinism: A group of inherited conditions that typically appear as a reduction or absence of melanin pigments in the skin, hair, and eyes.

albumin: A protein made by the liver. Abnormal levels of this substance may indicate liver or kidney disease.

albuminuria: High amounts of albumin (a protein made by the liver) in the urine, possibly indicating kidney dysfunction.

alcohol abuse: Continuing consumption of alcohol despite alcohol-related social or interpersonal problems.

alcohol dehydrogenase: A liver enzyme that metabolizes alcohol into a substance called acetaldehyde, which is toxic. Sometimes referred to as ADH.

alcohol dependence: A chronic, progressive disease characterized by excessive and often compulsive drinking, impaired control over drinking, use of alcohol despite adverse consequences, and withdrawal symptoms when alcohol use is discontinued. Also known as alcoholism.

alcoholism: Another term for alcohol dependence: A chronic, progressive disease characterized by excessive and often compulsive drinking, impaired control over drinking, use of alcohol despite adverse consequences, and withdrawal symptoms when alcohol use is discontinued. Also known as alcoholism.

aldosterone: A hormone secreted by the adrenal glands that helps regulate blood pressure by controlling sodium and potassium levels in the body.

alendronate: A drug used to treat and prevent osteoporosis by slowing bone loss.

alimentary canal: Another term for the gastrointestinal, or digestive, tract.

allele: One of two or more versions of a gene. Different alleles produce variations in inherited characteristics, such as eye color.

allergen: A substance such as fur, pollen, or dust that produces an allergic reaction.

allergic: Having a sensitivity to one or more normally harmless substances.

allergic rhinitis: A 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.

allergy: An immune system reaction (for example, rash, fever, sneezing, or headaches) to something that is normally harmless.

allodynia: Pain resulting from something not normally painful, such as a light touch.

alopecia areata: An autoimmune condition that appears as patchy hair loss on the scalp that may result in permanent hair loss.

alopecia totalis: Hair loss that involves the entire scalp.

alopecia universalis: Hair loss that involves the entire body.

alpha blockers: A group of drugs that lower blood pressure by blocking the effects of adrenaline or adrenaline-like substances on cells' alpha receptors. Also used to treat some prostate gland problems. Alpha blockers are also known as alpha-adrenergic antagonists, alpha-adrenergic blocking agents, and alpha-adrenergic blockers.

alpha cells: Cells in the pancreas that produce the hormone glucagon.

alpha hydroxy acids: Fruit-derived acids used in creams and lotions to act as exfoliants.

alpha waves: A type of brain wave generated when a person is relaxed, awake, and receiving no visual input (eyes closed or in the dark).

alpha-delta sleep: Abnormal deep sleep; also called non-restorative sleep.

alpha-glucosidase inhibitor: A drug used to treat Type 2 diabetes.

alveolar bone: The part of the jawbone that supports the teeth.

alveoli: Tiny air sacs in the lung. They are where oxygen enters and carbon dioxide leaves the bloodstream.

Alzheimer's disease: A progressive brain disease that causes memory loss, impaired thinking, and personality changes.

ambulatory: Able to walk; not confined to a bed.

AMD: Abbreviation for age-related macular degeneration, a potentially blinding condition that destroys sharp central vision.

amnesia: Unusual memory loss or forgetfulness.

amputation: The surgical removal of a limb or other body part.

Amsler grid: A tool used to check for vision problems, particularly macular degeneration. The grid looks like graph paper with a dot in the center.

amygdala: Part of the brain involved in memory and emotion.

amylase: An enzyme secreted by the pancreas that breaks starch into sugar.

amyloid: A protein that collects in tissues when certain diseases, including Alzheimer's disease, are present.

anaerobic: Any process that doesn't require oxygen. Often refers to a form of short, high intensity exercise, known as anaerobic exercise.

anaerobic exercise: Exercise that improves the efficiency of energy-producing systems that do not rely on oxygen. Examples include sprinting and weight lifting.

anagen: The active growth phase of the hair-growth cycle.

anal canal: The last inch of the large intestine, leading to the anal opening.

analgesia: Absence of pain.

analgesic: A drug or other substance such as aspirin, acetaminophen, or morphine that is used to relieve pain.

analytic variability: Differences in how a test is done, for example how a sample is prepared, which can affect test outcomes.

anaphylaxis: A severe allergic reaction, causing symptoms spanning from itching and swelling to trouble breathing, convulsions, shock, and coma.

androgen: Any of a group of male sex hormones, including testosterone, that controls male characteristics such as beard growth.

androgenetic alopecia: Female- and male-pattern baldness. The condition appears to involve a heightened response by the hair follicle to androgen levels in the body.

androgen-independent prostate cancer: Prostate cancer that no longer responds to hormone therapy.

anemia: Having a lower than normal amount of red blood cells or hemoglobin (an oxygen-carrying protein inside red blood cells), leading to low energy, weakness, and other symptoms.

anencephaly: A birth defect in which an infant is born without most of the brain or without the skull bones covering the brain.

anesthesia: Temporarily blocking sensation, especially the feeling of pain.

aneurysm: A bulge or swelling on a portion of a blood vessel, due to weakness in the wall of that vessel.

angina pectoris: Temporary chest pain that occurs when the heart isn't getting enough oxygen and blood, usually occurring in response to physical activity or stress.

angiogenesis: The formation of new blood vessels.

angiography: A test that shows how blood moves through the blood vessels and heart. It uses x-rays and the injection of a fluid called a contrast agent that can be seen on the x-rays.

angioplasty: A procedure used to open blocked or narrowed arteries, most commonly by inserting a thin tube, or catheter, into the affected artery and inflating a balloon.

angiotensin: A protein that raises blood pressure by narrowing blood vessels and causing the kidneys to store more sodium and water.

angiotensin I: An inactive form of the protein angiotensin. It is the precursor to the active form, angiotensin II.

angiotensin II: The active form of the protein angiotensin, which raises blood pressure by narrowing blood vessels and causing the kidneys to store more sodium and water.

angiotensin II receptor blockers: A class of drugs that blocks the effects of angiotensin. Like ACE inhibitors, they keep coronary arteries open, lower blood pressure, and reduce the heart's workload.

angiotensin-converting enzyme: An enzyme that converts the inactive form of the protein angiotensin (angiotensin I) to its active form—angiotensin II. Usually abbreviated as ACE.

angiotensin-converting enzyme inhibitor: A drug used to treat high blood pressure and congestive heart failure. These drugs stop the production of angiotensin II. This lowers blood pressure and reduces the heart's workload. Usually abbreviated as ACE inhibitor.

ankle-brachial index: A test that compares blood pressure at the ankle with blood pressure at the elbow. A difference between the two indicates the presence of peripheral artery disease.

ankylo-: Means crooked or bent; refers to stiffening of a joint.

ankylosing spondylitis: A disease that leads to swelling between the disks of the spine and in the joints where the pelvis and spine meet. Causes back pain and stiffness and can limit movement.

annulus: Term used to describe ring or circle shaped objects or body parts.

annulus fibrosus: The tough outer covering of the discs in the spine.

anorectal dysfunction: Abnormal functioning of the anus and rectum, causing constipation or the inability to control bowel movements.

anorexia: An eating disorder in which a person has an intense fear of gaining weight and severely limits calories to the point of near starvation.

antagonist: The muscle opposing the major muscle required to do a task. It works to help balance movement and ward off injury.

anterior myocardial infarction: A heart attack affecting the front of the heart.

antiandrogen: A drug that blocks or interferes with the activity of male sex hormones.

antibiotic: A substance that kills or slows the growth of bacteria.

antibody: A protein made by the immune system to protect the body from harmful substances, called antigens.

anticoagulant: A substance that helps prevent blood from clotting.

anticoagulants: Drugs that diminish the blood's ability to clot. Anticoagulants are sometimes called blood thinners even though they do not thin the blood. Commonly used anticoagulant drugs include heparin and warfarin.

anticonvulsants: Drugs used to treat seizures.

anti-cyclic citrullinated peptide: An antibody used to help diagnose rheumatoid arthritis.

antidepressant: A psychiatric drug used to treat mood disorders, such as depression.

antiemetic: A drug that prevents nausea and vomiting.

antigen: Any substance that the body sees as harmful or foreign, causing the immune system to form antibodies in defense.

antigen-presenting cell: Specialized white blood cells that detect harmful substances in the body and then signal other immune system defenders (known as T-cells) to mount a defense.

antihistamine: Medications that treat allergies and reduce symptoms such as sneezing and itching by blocking histamine, the substance in the body which causes these symptoms.

antihypertensives: Medications used to lower and control high blood pressure.

antileukotriene: A type of asthma medication that reduces swelling in airways and prevents muscles near the airways from tightening.

antimicrobial: A general term for antibiotics and other drugs that fight microscopic organisms in the body, such as bacteria, viruses, fungi, and parasites.

antioxidant: Substances that protect the body from molecules that damage cells (free radicals); examples include beta carotene, vitamin C, vitamin A, and vitamin E.

antiplatelet agents: Medications or other substances that prevent blood cells called platelets from clustering and forming blood clots.

antipsychotic: A drug used to treat schizophrenia and other severe mental health disorders; relieves symptoms such as delusions and hallucinations.

antiseptic: Substances used on wounds to prevent or treat infection; they kill or slow the growth of disease-causing organisms, such as bacteria, on the surface of the body.

antispasmodic drugs: Drug that relieves cramps and muscle spasms.

antithyroid drugs: Drugs used to treat an overactive thyroid.

anus: The external opening at the end of digestive tract where feces are expelled.

aorta: The large artery emerging from the heart's left ventricle that distributes blood to the body.

aortic valve: A valve on the left side of the heart that acts as a one-way gate, opening to allow blood to leave the left ventricle and closing to prevent blood from leaking back into that ventricle.

aphasia: Difficulty speaking or comprehending language; a common occurrence after a stroke affecting the left hemisphere of the brain, where language is processed.

apnea: A temporary pause in breathing during sleep that can be very brief or can last so long that the amount of oxygen in the blood drops dangerously low.

apolipoproteins: Proteins that combine with cholesterol and triglyceride to form lipoproteins.

apoptosis: A process of programmed cell death in which redundant or flawed cells destroy themselves.

amyloid precursor protein: A normal brain protein that under certain circumstances produces beta amyloid, abnormal protein deposited in the brain in Alzheimer's disease. Usually abbreviated as APP.

apraxia: A brain disorder in which a person cannot perform certain actions, such as combing hair, picking up a pencil, or speaking, even though they want to and have the physical ability to do so.

aqueous humor: Clear fluid that fills the front part of the eye.

ARB: Abbreviation for angiotensin II receptor blockers, a class of drugs that blocks the effects of angiotensin. Like ACE inhibitors, they keep coronary arteries open, lower blood pressure, and reduce the heart's workload.

arbovirus: A virus transmitted by mosquitoes or other member of the arthropod phylum.

arousal: The state of being awake or reactive to stimuli through one or more of the five senses.

arrector pili: The small muscle associated with an individual hair follicle that enables hair to stand on end.

arrhythmia: An abnormal heart rhythm caused by a disturbance in the heart's electrical system.

arterial resistance: The pressure that the artery walls exert on blood flow; in general, the less elastic the arteries, the greater the arterial resistance and the higher the blood pressure.

arteriography: A test that uses x-rays and a special dye to see inside arteries.

arterioles: Small, muscular branches of arteries.

arteriosclerosis: A term encompassing a variety of conditions in which artery walls thicken and become less flexible. Sometimes called hardening of the arteries. Arteriosclerosis occurs when cholesterol-rich plaque forms on the inner lining of arteries (atherosclerosis), when artery walls become calcified, or when high blood pressure thickens the muscular wall of arteries.

arteriovenous malformation: Abnormal connections between veins and arteries, usually caused by a birth defect.

artery: A blood vessel that carries blood away from the heart and to various parts of the body.

arthr-: A prefix meaning "joint.''

arthritis: A condition in which joints are inflamed, causing pain, stiffness, swelling, and sometimes loss of movement.

arthrocentesis: A procedure to drain fluid from a joint using a syringe.

arthrodesis: Joining together two bones to reduce pain and provide stability to a damaged, arthritic, or painful joint.

arthropathy: Joint disease or disorder.

arthroplasty: Surgically rebuilding or replacing a joint, usually to relieve arthritis or fix an abnormality.

arthroscopy: A procedure where a surgeon makes a small cut in the skin and inserts tiny lenses, lighting, and other instruments to diagnose or repair joint problems.

articular cartilage: Smooth white tissue that covers the ends of bones where they come together to form joints.

articular processes: Bony projections on vertebra.

asphyxia: A life-threatening lack of oxygen due to drowning, choking, or an obstruction of the airways.

aspiration: Breathing in a foreign object. Also, the process of suctioning fluid, tissue, or other substances from the body.

aspirin: A drug that relieves pain, fever, and swelling, and inhibits the formation of blood clots.

assisted living: Live-in facilities for adults who need help with certain things, but do not need round-the-clock care. They provide residents with supervision and certain services, such as meals, transportation, or help with dressing, grooming, and other daily activities.

association cortex: The part of the cerebral cortex involved in processing information, rather than movement or sensory experiences.

asthma: A disease that inflames and narrows airways, causing wheezing, shortness of breath, coughing, and tightness in the chest.

astigmatism: Blurry vision caused by an irregular curve in the cornea of the eye.

astringent: A substance that contracts skin tissues and shrinks pores.

asymptomatic: Showing no signs or symptoms of disease, whether or not disease is present.

asystole: The absence of electrical activity in the heart.

ataxia: Being unable to control movement; symptoms include shaking and an unsteady walk.

atheroma: An abnormal build-up of fatty plaque inside an artery.

atherosclerosis: The buildup of fatty deposits (plaque) in the walls of arteries, causing narrowing and reduced blood flow; the disease responsible for most heart attacks and many strokes.

atherosclerotic plaque: A mixture of fats, cholesterol, and other tissue that builds up on artery walls.

atherothrombotic stroke: A type of stroke that occurs when a large artery to the brain is completely blocked by the formation of a clot.

athlete's foot: A foot infection caused by fungus; symptoms include cracking and peeling skin and itchiness. Also known as tinea pedis.

atlas: Another name for the topmost vertebra of the neck, which lies just beneath the skull. Also called C-1.

atopic: Having an inherited predisposition to allergies.

atopic dermatitis: A long-term skin condition, most common in babies and children, in which areas of the skin are dry, itchy, red, and may crack. Also known as eczema.

atopic rhinitis: A seasonal or year-round allergy marked by sneezing, runny nose, and congestion.

atopy: The inherited tendency to develop allergies.

ATP: Abbreviation for adenosine triphosphate, an energy-storing molecule that is found in all human cells.

atria: The upper chambers of the heart. There are two of these—the right atrium and the left atrium.

atrial fibrillation: A disorder in which the two upper chambers of the heart beat fast and erratically. Because blood isn't pumped out of these chambers fully, it may pool and form clots that could lead to a stroke.

atrioventricular node: Also known as the AV node. A major part of the electrical system in the heart that acts as a gateway between the atria and the ventricles. An electrical signal generated by the sinoatrial node (the heart's natural pacemaker) moves through the heart until it reaches the atrioventricular node, a cluster of cells at the bottom of the right atrium. The AV node delays the signal before it is passed to the ventricles. This lets the atria fully contract before the ventricles contract.

atrium: One of the two upper chambers of the heart.

atrium: One of the heart's two upper chambers (the plural form is atria). The right atrium receives deoxygenated blood from the body; the left atrium receives oxygenated blood from the lungs.

atrophy: Wasting away of an organ or tissue due to undernourishment, disease, or aging.

atypical lobular hyperplasia: An overgrowth of abnormally shaped cells in areas of the breast that produce milk (lobules). atypical lobular hyperplasia is not cancerous but may become cancer.

audiogram: A chart that shows a person's ability to hear at different pitches or frequencies.

audiologist: A health professional who assesses hearing and fits hearing aids.

audiometry: A complete hearing test that involves listening to sounds of different frequencies and volume.

auditory nerve: A nerve in the inner ear that transmits information about sound to the brain.

aura: Sensations such as chills, flashes of light, or a blind spot that come just before the occurrence of medical problems such as migraines or seizures.

autoantibodies: Proteins created by the immune system that mistakenly target healthy cells, tissues, or organs.

autoimmune disease: A disease in which the immune system mistakenly identifies healthy tissues and organs as threats and responds by attacking and destroying them.

autoimmune response: When the body's immune system mistakenly views the body's own tissues and organs as foreign invaders and attacks them.

autologous fat transplant: Removal of fat from one part of the body to use as filler in another part, for example, to fill wrinkles and lines in the face and lips.

autonomic nervous system: The part of the nervous system that controls involuntary actions, such as blood pressure or breathing. It also plays an important role in the fight or flight response to danger.

autonomic neuropathy: Damage to the nerves that control involuntary body functions, such as digestion, heart rate, and bladder and bowel function.

autopsy: Surgically opening and examining a body after death to see if any diseases are present and to determine the cause of death.

AV node: Abbreviation for atrioventricular node, a major part of the electrical system in the heart that acts as a gateway between the atria and the ventricles. An electrical signal generated by the sinoatrial node (the heart's natural pacemaker) moves through the heart until it reaches the atrioventricular node, a cluster of cells at the bottom of the right atrium. The AV node delays the signal before it is passed to the ventricles. This lets the atria fully contract before the ventricles contract.

avulsion: The tearing away of one part of the body from another—for example, a tendon tearing away from a bone.

axillary: The armpit.

axis: The second vertebra of the neck (from the skull); also called the C-2 vertebra.

axon: The long, slender extension of a nerve cell that conducts electrical impulses away from the nerve's cell body and on to nearby nerves.

axon terminal: The end of an axon.