labile hypertension: Blood pressure that frequently fluctuates between normal and abnormal during the course of a day, often within only a few minutes.

labyrinth: The inner ear. It contains the cochlea, which is responsible for hearing, as well as structures that are needed for balance.

laceration: A tear in the skin.

lacrimal gland: The gland that produces tears.

lactase: An enzyme that breaks down milk sugar (lactose) in the body.

lactic acidosis: A rare but potentially lethal condition in which blood lactic acid levels increase.

lactose: A sugar found in milk and dairy products.

lactose intolerance: The inability of the body to easily digest lactose, a sugar found in milk and dairy products.

lacunar stroke: A small ischemic stroke caused by the blockage of one of the smaller blood vessels in the brain; the most common effect is weakness or disability on one side of the body.

lamellar bone: Hard, dense tissue that forms the outer shell of bones. Also called compact bone.

lamina: One of the two thin, plate-like parts of each vertebra.

laminectomy: An operation in which all or part of one or both laminae is removed.

Langerhans cells: Cells of the immune system that work in the skin to fight infection.

lanugo: Fine, soft hair that grows all over the body of a fetus and is typically shed before birth.

laparoscopy: A surgical procedure carried out with tiny instruments inserted through small openings in the skin.

laser: A concentrated beam of light; lasers are used in certain surgeries and other medical procedures.

laser assisted uvula palatoplasty: A surgical procedure to ease snoring by removing or reshaping some of the tissues in the mouth (usually the uvula and soft palate) that vibrate and cause the noise of snoring.

laser hair removal: Permanent hair removal technique that uses a laser to target and heat melanin in the hair shaft, which damages the hair follicle.

laser photocoagulation: Surgery that uses a laser to seal off blood vessels in the eye; used to treat a number of eye diseases.

laser prostatectomy: A surgical technique for treating an enlarged prostate (benign prostatic hyperplasia). It uses a high-energy laser to remove large amounts of prostate tissue with little bleeding.

latex allergy: An allergic reaction to the proteins found in natural rubber.

laxative: A drug or substance that induces bowel movements or makes the stool softer and looser.

LDL: Abbreviation for low-density lipoprotein. This so-called bad cholesterol can build up on artery walls, narrowing the artery and making a heart attack or stroke more likely.

learned insomnia: When fear of not sleeping develops after a short period of not sleeping well, and this anxiety causes ongoing trouble falling or staying asleep.

LED photomodulation: Use of a panel of light-emitting diodes (LEDs) to stimulate skin cells and improve the look of the skin.

left anterior descending coronary artery: One of the main arteries that supplies blood to the heart; it runs down the front surface of the heart.

left atrium: The left upper chamber of the heart.

left circumflex coronary artery: One of the main arteries that supplies blood to the heart; it curves around the back of the heart.

left ventricle: The left lower chamber of the heart. It pumps blood out of the heart to other organs in the body.

left ventricular hypertrophy: A thickening of the wall of the lower left chamber of the heart, which is responsible for pumping blood to organs throughout the body.

left-ventricular assist device: A surgically implanted pump that augments the pumping action of the left ventricle.

lensometer: A device used to check the prescription of eyeglasses.

leptin: A hormone produced by fat cells that acts on the brain to suppress appetite and burn stored fat.

lesion: An infected, diseased, or wounded area of tissue.

leukotriene blockers: Asthma medications that work by blocking leukotrienes, chemicals made in the body as part of an allergic reaction.

leukotriene modifiers: Asthma medications that work by blocking the production or action of leukotrienes, chemicals made in the body as part of an allergic reaction.

leukotrienes: Chemicals that cause airways to swell when an allergic reaction occurs or in diseases like asthma.

levothyroxine sodium: A man-made version of the thyroid hormone thyroxine.

LH: Abbreviation for luteinizing hormone, a hormone that controls ovulation in women. In men, luteinizing hormone triggers production of testosterone.

libido: Sexual desire.

lice: A blood-sucking parasite known as Pediculus humanus capitis that can cause an itchy scalp; infestations are highly contagious and especially common in school-age children.

ligament: A band of tissue that connects bones.

ligature: Any material that is tied around a blood vessel to stop it from bleeding.

lignans: Antioxidant chemicals found in seeds like flax and sesame, as well as some fruits, vegetables, and grains.

limbic system: A group of structures in the brain that help control memory, emotions, sexual arousal, and motivation.

limbus: The border between the cornea and the white of the eye.

liothyronine sodium: A man-made version of the thyroid hormone triiodothyronine.

lipase: An enzyme secreted by the pancreas that helps the body break down fats.

lipids: Fats, oils, and waxes that serve as building blocks for cells or as energy sources. Lipids are also capable of accumulating in the artery walls to form plaque.

lipoatrophy: Dents or depressions in the skin that are caused by a loss of fatty tissue.

lipohypertrophy: A buildup of fatty tissue.

lipoma: A noncancerous tumor or growth composed of fat cells.

lipoprotein: A combination of fat (lipid) and protein molecules bound together as packages. The combination allows fats and cholesterol to move easily through the blood.

lipoprotein analysis: A test that measures the amount of triglycerides, total cholesterol, HDL (good cholesterol), and LDL (bad cholesterol) in a person's blood.

lipoprotein(a): A molecule made up of fat and protein that is very similar to harmful LDL.

liposuction: A cosmetic procedure that removes fat from an area of the body.

live attenuated vaccines: Vaccines made from a weakened virus or microbe. These weakened viruses and microbes don't cause disease; instead they teach the body to recognize the substance as harmful and destroy it if there is contact with it in the future.

liver: A vital organ that removes waste products from the body and helps with digestion.

living will: A legal document that states what a person would and wouldn't want if he or she is no longer able to make health care decisions.

lobar hemorrhage: Bleeding that occurs in the white matter of the brain beneath the cerebral cortex.

lobotomy: A surgical procedure to severe one or more branches of nerves into the frontal lobe of the brain.

lobules: Milk-producing glands of the breast.

localized: In reference to cancer, generally means cancer that is limited to a specific gland or other tissue, without any distant spread; an organ-confined cancer.

locus ceruleus: An area of the brain stem that helps control the brain's alertness, responses to certain stimuli, and stress and anxiety levels.

long-term memory: A memory that lasts from a few minutes to decades.

low-calorie diet: A weight-loss plan that limits calories to 800–1,500 a day.

low-density lipoprotein: So-called bad cholesterol. If there is too much LDL in the blood, it can collect on artery walls, narrowing them and making heart attacks and strokes more likely.

lower esophageal sphincter: A ring of muscle where the esophagus and stomach meet. It relaxes to let food into the stomach and closes to prevent stomach acids from backing up and irritating the esophagus.

lumbar puncture: A procedure in which a hollow needle is inserted into the lower part of the spinal canal to withdraw fluid for testing. Also called a spinal tap.

lumbar spine: The lower portion of the spine. It includes the five lowermost bones (vertebrae) of the spine and is the start of the lower back.

lumbar stenosis: Narrowing of the spinal canal in the lower portion of the spine (known as the lumbar spine).

lumen: The hollow part of a tube, like a blood vessel, or the cavity in a hollow organ.

lutein: A natural substance found in green leafy vegetables.

luteinizing hormone: A hormone that controls ovulation in women. In men, luteinizing hormone triggers production of testosterone.

luteinizing hormone-releasing hormone agonist: A drug that slows the production of hormones by the testicles in men or the ovaries in women.

LVAD: Abbreviation for left-ventricular assist device, a surgically implanted pump that augments the pumping action of the left ventricle.

Lyme disease: An infectious disease transmitted by a tick bite.

lymph nodes: Small, bean-shaped organs that filter germs and foreign matter out of the body. Also called lymph glands.

lymphatic system: A system of channels that drains excess clear fluid, called lymph, from tissues and returns it into the bloodstream.

lymphedema: Blockage in or damage to the lymphatic systems, causing lymphatic fluid to build up in tissues, making them swell.

lymphocyte: A type of white blood cell that can recognize foreign substances in the body.

lymphoma: A type of cancer that affects cells in the lymphatic system.