balance - biological system that enables individuals to know where their bodies are in the environment and to maintain a desired position; normal balance depends on information from the labyrinth in the inner ear, and from other senses such as sight and touch.

balance disorder - disruption in the labyrinth - the inner ear organ that controls the balance system allowing individuals to know where their bodies are in the environment.

balloon angioplasty (Also called coronary artery balloon dilation.) - a procedure used to widen narrowed arteries. The procedure uses a catheter with a deflated balloon at the tip that is inserted into the narrowed part of the artery. The balloon is then inflated, causing the artery to dilate (widen).

barium - a metallic, chemical, chalky, liquid used to coat the inside of organs so that they will show up on an x-ray.

barium swallow - upper gastrointestinal (GI) series.

barotrauma - injury to the middle ear caused by a reduction of air pressure.

basal body temperature - temperature of a person's body taken first thing in the morning after several hours of sleep and before any activity, including getting out of bed or talking; often charted to determine the time of ovulation.

basal ganglia - several large clusters of nerve cells, including the striatum and the substantia nigra, deep in the brain below the cerebral hemispheres.

basal metabolic rate (BMR) - a measurement of energy required to keep the body functioning at rest. Measured in calories, metabolic rates increase with exertion, stress, fear, and/or illness.

base of the lung - bottom portion of lower lobes of the lungs, located just above the diaphragm. 

beta blocker - an antihypertensive medication that limits the activity of epinephrine (a hormone that increases blood pressure).

beta cells - cells that make insulin, found in areas of the pancreas called the islets of Langerhans.

biarthrodial muscles - muscles that span over two joints and have a function over those joints.

bilateral - affecting both sides of the body; for example, bilateral breast cancer is cancer occurring in both breasts at the same time (synchronous) or at different times (metachronous).

biliary tract (Also called biliary system or biliary tree.) - gallbladder and the bile ducts.   

binocular vision - the ability to use both eyes at once.

bladder - a hollow organ that urine drains into from the kidneys.

blink rate - the number of times per minute that the eyelid automatically closes - normally 10 to 30 times per minute. 

blood plasma - the fluid part of blood that contains nutrients, glucose, proteins, minerals, enzymes, and other substances.

blood pressure cuff - a device usually placed around the upper part of the arm to measure blood pressure.

blood-brain barrier - the protective membrane that separates circulating blood from brain cells.  

body mass index (BMI) - a number, derived by using height and weight measurements, that gives a general indication of whether or not weight falls within a healthy range.

bone - living tissue that makes up the body's skeleton. 

bone density test - a test that measures the strength and density of bones; often used to determine the risk of developing osteoporosis.

borborygmi (Also called stomach "growling".) - rumbling sounds caused by gas moving through the intestines.

botanical - of plants and plant life.

bowel - another word for the small and large intestines.

bowel movement - body wastes passed through the rectum and anus.

bowel prep - process used to clean the colon with enemas and a special drink; used before surgery of the colon, colonoscopy, or barium x-ray and other imaging exams. (See also lavage.)

bradycardia - abnormally slow heartbeat.  

bradykinesia - slowness of movement.

bradyphrenia - slowness of thought processes.

brain attack - another name for stroke.

brain scan - an imaging method used to find abnormalities in the brain, including brain cancer and cancer that has spread to the brain from other places in the body.

brainstem implant - auditory prosthesis that bypasses the cochlea and auditory nerve to help individuals who cannot benefit from a cochlear implant because the auditory nerves are not working.

Braxton Hicks contractions - relatively brief, painless contractions of the uterus that may begin during the second half of pregnancy. 

BRCA1 - a gene, which, when damaged or mutated, places a woman at much higher risk of developing breast and/or ovarian cancer than the general population.

BRCA2 - a gene, which, when damaged or mutated, places a woman at a much higher risk for developing breast cancer and/or ovarian cancer than the general population.

breast cancer - a cancerous tumor of the breast tissue.

breast conservation therapy - surgery to remove a breast cancer and a small amount of benign tissue around the cancer without removing any other part of the breast. This procedure is also called lumpectomy, segmental excision, limited breast surgery, or tylectomy.

breast implant - a manufactured sac that is filled with silicone gel (a synthetic material) or saline (sterile saltwater); the sac is surgically inserted to increase breast size or restore the contour of a breast after mastectomy.

breast reconstruction - surgery to rebuild a breast mound after mastectomy.

breast specialist - term describing health care professionals who have a dedicated interest in breast health. While they may acquire specialized knowledge in this area, medical licensing boards do not certify a specialty in breast care.

bronchodilators - a group of drugs that widen the airways in the lungs.

bronchus (pl. bronchi) - one of the two large airways of the lungs. The trachea divides into the right and left bronchi.

Budd-Chiari syndrome - a rare liver disease in which the veins that drain blood from the liver are blocked or narrowed.

bulking agents - laxatives that make bowel movements soft and easy to pass.

bundle-branch block - a condition in which the heart's electrical system is unable to normally conduct the electrical signal.