M

| A || B || C || D || E || F || G || H || I || J || K || L || M |
| N || O || P || Q || R || S || T || U || V || W || X || Y || Z |

Macrodactyly - a congenital problem in which there is an abnormal growth of a finger or toe.

Macrosomia - a condition in which a baby is considerably larger than normal at birth.

Macula - a small spot inside the eye on the retina that provides the most visual detail.

Macular degeneration - an eye condition in which degeneration in the macular region of the retina results in decreased central vision and, sometimes, blindness.

Macular stains (also called angel's kisses or stork bites) - faint, red marks that appear on the skin at birth. Angel's kisses are marks on the forehead and eyelids. Stork bites are marks on the back of the neck.

Macule - the smaller version of a patch; a flat, discolored spot.

Mad cow disease (Also called bovine spongiform encephalopathy or BSE) - a neurological disease in cattle that is related to a disease in humans called new variant Creutzfeldt-Jakob disease (nvCJD). Both disorders are fatal brain diseases caused by an unconventional transmissible agent.

Magnetic resonance imaging (MRI) - a diagnostic procedure that uses a combination of large magnets, radiofrequencies, and a computer to produce detailed images of organs and structures within the body.

Major depression (also known as clinical depression or unipolar depression) - a type of affective disorder or mood disorder in which a person's ability to function in normal activities is affected by feelings of sadness, despair, loss, anger, and/or frustration over a long period of time.

Malabsorption syndromes - conditions that may result when the small intestine cannot absorb nutrients from foods.

Malaria - a disease caused by a parasite that is transmitted by the bite of an infected Anopheles mosquito. These mosquitoes are present in the tropics and subtropics in almost all countries. Malaria is the most deadly of all tropical parasitic diseases.

Malignant - cancerous cells are present.

Malignant melanoma - a rare, but sometimes deadly, skin cancer that begins as a mole that turns cancerous.

Malignant tumor - a mass of cancer cells that may invade surrounding tissues or spread to distant areas of the body.

Malnutrition - condition caused by not eating enough food or not eating a balanced diet.

Malocclusion - an orthodontic problem that means "bad bite," including crowded, missing, or crooked teeth, extra teeth, or a misaligned jaw.

Mammogram - X-ray of the breast tissue.

Mania - a mood disorder which may be characterized by extreme elation, impulsivity, irritability, rapid speech, nervousness, distractibility, and/or poor judgment.

Manic depression (also known as bipolar disorder) - a type of affective disorder or mood disorder that is characterized by periodic episodes of extreme elation, elevated mood, or irritability (also called mania) countered by periodic, classic depressive symptoms.

Manometry - tests that measure pressure using a device called a manometer. Blood pressure is measured with a sphymomanometer; pressure inside the esophagus or anus can also be measured by manometry.

Mantoux test - a skin test used to identify the presence of tuberculosis infection.

Manual therapy - a type of physical therapy that uses only manual (hands on) techniques.

Massage - mechanical form of therapy in which the soft tissues are made more pliable, promoting increased blood flow and healing.

Mast cell - a type of cell that is involved in inflammatory responses and allergic reactions in the body. When mast cells are stimulated by an allergen, they release histamine and other substances.

Mastalgia - pain in the breast that is generally classified as either cyclical (associated with menstrual periods) or non-cyclical.

Mastectomy - surgical removal of all or part of the breast.

Mastitis - an inflammation of the breast tissue.

Mastoid - back portion of the temporal bone behind the ear.

Mastoid surgery - a type of surgical procedure in which part of the mastoid bone is removed because of infection or other reasons.

Maxillofacial - pertaining to the jaws and face.

Meal plan - a guide to help people include the proper amount of calories, carbohydrates, proteins, fats, vitamins, minerals, and fiber in their diet.

Mean blood pressure - average blood pressure, taking account of the rise and fall that occurs with each heartbeat. It is often estimated by multiplying the diastolic pressure by two, adding the systolic pressure, and then dividing this sum by three.

Measles - a very contagious viral illness characterized by a distinct rash and a fever; spread through airborne droplets of nasal secretions.

Meckel's diverticulum - a birth defect in which a small sac that forms in the ileum (lower end of the small intestine) before birth does not resolve by the time the baby is born.

Meconium - a sticky, greenish-black substance that forms in the intestines during fetal development and is the first bowel movement of a newborn.

Medial collateral ligament (MCL) - the ligament that gives stability to the inner knee.

Medial epicondylitis (also known as golfer's elbow, baseball elbow, suitcase elbow, or forehand tennis elbow) - pain caused by inflammation of the tendons connected to the muscles that bend the wrist toward the palm.

Median nerve - large nerve, comprising segments from the cervical spine, that is involved in nerve function of the upper limb; commonly compressed in the carpal tunnel of the wrist.

Medicaid - federally and state-funded health care program for low-income individuals.

Medicare - federally funded health care program for individuals over age 65.

Megacolon - enlarged colon; may be caused by severe constipation. Hirschsprung's disease is a congenital (present at birth) type of megacolon that occurs because of a lack of nerves in parts of the colon.

Megaloblastic (pernicious) anemia - a rare blood disorder in which the body does not absorb enough vitamin B12 from the digestive tract, resulting in an inadequate amount of red blood cells produced.

Meige syndrome - a rare movement disorder that can involve excessive eye blinking (blepharospasm) with involuntary movements of the jaw muscles, lips, and tongue (oromandibular dystonia).

Melanin (also called pigment) - a substance that gives the skin its color.

Melanocytes - cells present in the epidermis that produce melanin (skin pigment).

Melanoma - the most serious, life-threatening form of skin cancer.

Melasma - dark, brown, symmetrical patches of pigment on the face.

Melena - blood in the stool.

Menarche - a young woman's first menstrual period.

Ménétrier disease (also called hypoproteinemic hypertrophic gastropathy) - a condition in which the ridges inside the stomach, called ruguae, become enlarged and cause too much mucus to collect in the stomach. The stomach also absorbs protein from the blood, causing too little protein in the blood. There is also a decrease in acid production in the stomach.

Ménière's disease - an inner ear disorder that can affect both hearing and balance; can cause vertigo, hearing loss, tinnitus, and the sensation of fullness in the ear.

Meningitis - inflammation of the meninges, the membranes that envelop the brain and the spinal cord.

Menisci - two crescent-shaped discs of connective tissue between the bones of the knees that act as shock absorbers to cushion the lower part of the leg from the weight of the rest of the body.

Menopause - end of menstruation; commonly used to refer to the period ending the female reproductive phase of life.

Menorrhagia (also called dysfunctional uterine bleeding) - the most common type of abnormal uterine bleeding characterized by heavy and prolonged menstrual bleeding. In some cases, bleeding may be so severe and relentless that daily activities become interrupted.

Menses - menstrual flow.

Menstruation - a cyclical process of the endometrium shedding its lining, along with discharge from the cervix and vagina, from the vaginal opening. This process results from the mature egg cell (ovum) not being fertilized by a sperm cell as it travels from one of the ovaries down a fallopian tube to the uterus, in the process called ovulation.

Mercury poisoning - an abnormal condition caused by breathing or swallowing a mercury substance.

Metabolism - the chemical activity that occurs in cells, releasing energy from nutrients or using energy to create other substances, such as proteins.

Metaplasia - the phenomenon by which one type of tissue is replaced by another type.

Metastasis - the spread of cancer cells to distant areas of the body by way of the lymph system or blood stream.

Metastasize - when cancer cells spread to other parts of the body.

Methyl-tert-butyl ether (MTBE) - a solution injected into the gallbladder to dissolve gallstones. Use of MTBE for this purpose remains investigational and is not done widely. 

Metrorrhagia - any irregular, acyclic, non-menstrual bleeding from the uterus; bleeding between menstrual periods.

Micrographia - a change in handwriting with the script becoming smaller and more cramped.

Microsurgical fertilization - a procedure used to facilitate sperm penetration into the oocyte, and fertilization takes place under the microscope.

Middle ear - part of the ear that includes the eardrum and three tiny bones of the middle ear, ending at the round window that leads to the inner ear.

Milia - tiny, white, hard spots that look like pimples on a newborn's nose. Adults may also have milia on the face. 

Minimally invasive surgery - any technique involved in surgery that does not require a large incision.

Miosis - constriction of the pupil.

Misarticulation - inaccurately produced speech sound (phoneme) or sounds.

Miscarriage - spontaneous termination of a pregnancy before the fetus has developed enough to survive outside the uterus.

Mitral valve - the valve that controls blood flow between the left atrium and left ventricle in the heart.

Mitral valve prolapse - a bulge in the valve between the left atrium and left ventricle of the heart that causes backward flow of blood into the atrium.

Mobility - movement.

Modified radical mastectomy - the removal of the entire breast (including the nipple, areola, and overlying skin), some of the lymph nodes under the arm (also called the axillary lymph glands), and the lining over the chest muscles. In some cases, part of the chest wall muscles is also removed.

Mold - a microscopic fungus that grows and lives on plant or animal matter or on non-organic objects. Most molds are made up of filaments and reproduce through the production of spores, which spread by air, water, or insects.

Moles - small skin marks caused by pigment-producing cells in the skin.

Mongolian spots - bluish-black marks on the lower back and buttocks; affects mainly African-American or Asian children. The spots appear at birth or soon after, and generally disappear without treatment within a few years. 

Monoclonal antibodies - substances that can locate and bind to cancer cells wherever they are in the body.

Monounsaturated fats - dietary fats, such as olive oil or canola oil, that do not seem to have any affect on blood cholesterol.

Morning-after pills - hormonal medications to prevent pregnancy taken within 72 hours of having unprotected intercourse.

Moro reflex - movement of arms and legs that occurs when a newborn is startled by a loud sound or movement.

Morton's neuroma - a condition in which the nerve tissue between the toes, usually the third and fourth toes, becomes thickened, causing pain in the ball of the foot, as well as tingling, burning, and cramping between the toes.

Motility - movement of food through the digestive tract.

Motion sickness - dizziness, sweating, nausea, vomiting, and generalized discomfort experienced when an individual is in motion.

Motor speech disorders - group of speech disorders caused by the inability to accurately produce speech sounds (phonemes), because of weakness or lack of coordination of involved muscles.

Moulding - elongation of the shape of a baby's head due to delivery through the birth canal.

Moxibustion - the burning of herbal leaves on or near the body.

Mucosal lining - lining of gastrointestinal (GI) tract organs that makes mucus.

Mucosal protective drugs - medications that protect the stomach lining from acid.

Mucus - a thick, jelly-like substance made by the intestines and other organs of the body (such as the nose), that helps coat and protect the lining of the organ. Mucus also helps stool pass through the large intestine and rectum more easily.

Multifidus - deep lumbar spine muscle that stabilizes the lumbar spine.

Multiple chemical sensitivity - a diagnostic label for people who suffer multi-system illnesses as a result of contact with, or proximity to, a variety of airborne agents and other substances.

Multiple sclerosis (MS) - an autoimmune disease that affects the brain and spinal cord by damaging the sheath surrounding nerve cells. MS can be relatively benign, disabling, or devastating, leaving the patient unable to speak, walk, or write.

Multiple semen analysis - a test to evaluate a man's sperm in which at least two semen examples are collected on separate days to examine the semen and sperm for various factors, such as semen volume, consistency, and pH, and the sperm count, motility, and morphology (shape).

Mumps - an acute and highly contagious viral disease that usually occurs in childhood. Spread by airborne droplets from the upper respiratory tract, the disease usually takes two to three weeks to appear.

Murmur - a blowing or rasping sound heard while listening to the heart that may or may not indicate problems within the heart or circulatory system.

Muscular dystrophy (MD) - a broad term that describes a group of genetic (inherited) disorder of the muscles. MD causes the muscles in the body to become very weak. The muscles break down and are replaced with fatty deposits over time. The most common form of MD is called Duchenne muscular dystrophy (DMD).

Musculoskeletal system - the complex system involving the body's muscles and skeleton, and including the joints, ligaments, tendons, and nerves.

Mycoplasma genitalium - very common sexually transmitted disease or urinary tract infection caused by a bacteria-like organism in the urethra and reproductive system.

Mycosis fungoides - cutaneous T-cell lymphoma skin tumors.

Mydriasis - dilation of the pupil.

Myelogenous leukemia - a type of leukemia in which the cancer develops in the granulocytes or monocytes (myeloid cells).

Myelogram - involves the injection of a dye or contrast material into the spinal canal; a specific X-ray study that also allows careful evaluation of the spinal canal and nerve roots.

Myeloproliferative disorders - diseases in which the bone marrow produces too many of one of the three types of blood cells: red blood cells, which carry oxygen to all the tissues in the body; white blood cells, which fight infection; and platelets, which control bleeding.

Myocardial infarction (also called heart attack) - occurs when one of more regions of the heart muscle experience a severe or prolonged decrease in oxygen supply caused by a blocked blood flow to the heart muscle.

Myocardial ischemia - insufficient blood flow to part of the heart.

Myocardium - the muscle wall of the heart.

Myoclonus - jerking, involuntary movements of the arms and legs; may occur normally during sleep.

Myofascial pain - the most common form of temporomandibular disorder; discomfort or pain in the muscles that control jaw function and the neck and shoulder muscles.

Myomectomy - surgical procedure done to remove fibroids from the uterus while leaving the uterus intact.

Myopia - nearsightedness.

Myringotomy - a surgical opening of the eardrum to release pressure on the middle ear.