hair cells - sensory cells of the inner ear, which are topped with hair-like structures (stereocilia), which transform the mechanical energy of sound waves into nerve impulses.

halitosis - an oral health condition characterized by consistently odorous breath.

hallux rigidus - loss of range of the movement of the metatarsophalangeal (MTP) joint of the first (great) toe.  

hamstrings - muscles located in the posterior compartment of the thigh.  

hay fever - see rhinitis.

headache, primary - includes tension (muscular contraction), vascular (migraine), and cluster headaches not caused by other underlying medical conditions.

headache, secondary (Also called traction headaches or inflammatory headaches.) - includes headaches that result from other medical conditions.

hearing - series of events in which sound waves in the air are converted to electrical signals that are sent as nerve impulses to the brain where they are interpreted.

hearing aid - electronic device that brings amplified sound to the ear.

hearing disorder - disruption in the normal hearing process; sound waves are not converted to electrical signals and nerve impulses are not transmitted to the brain to be interpreted.  

heart block - interrupted electrical impulse to heart muscles.    

heart valve prolapse - a condition of the heart valve in which it is partially open when it should be closed.    

heartbeat - one complete contraction of the heart.

heart-lung machine - a machine that performs for the heart during open heart surgery.

Heberden's nodes - bony swellings around the margins of joints, associated with degenerative changes of arthritis.

hematopathology - the study of blood, bone marrow and the organs and tissues that use blood cells to perform their functions.

hemorrhage - the medical term for bleeding. 

hemorrhoidectomy - the surgical removal of hemorrhoids. 

hepatitis E - virus spread mostly through unclean water. This type of hepatitis is common in developing countries and has not occurred in the United States.

hepatology - field of medicine concerned with the functions and disorders of the liver.

high density lipoprotein (HDL) - the "good" cholesterol that promotes breakdown and removal of cholesterol from the body.

hip - the region on each side of the pelvis that is made up of three sections: ilium, ischium, and pubis; the upper part of the femur (upper leg bone) fits into the hip via a ball-and-socket joint; the socket is a cup-shaped bone of the pelvis called the acetab

histamine - a chemical present in cells throughout the body that is released during an allergic reaction.

hives - see urticaria

hormone replacement therapy (HRT) - use of the female hormones estrogen and progestin (a synthetic form of progesterone) to replace those the body no longer produces after menopause.

hormone therapy - the use of hormones, medications, or surgery to suppress (block) or mimic hormones and alter the growth of hormone-sensitive cancer.

hormones - chemical substances created by the body that control numerous body functions.

human papillomaviruses (HPVs) - a group of viruses that can cause warts. Some HPVs are sexually transmitted and cause wart-like growths on the genitals. HPV is a major risk factor for cervical cancer.  

hyaloid canal - narrow passageway that allows blood to flow through the eye.

hydrocortisone - a hormone secreted by the adrenal cortex which affects metabolism.  

hydrogen breath test - test for lactose intolerance that measures breath samples for too much hydrogen.

hydrotherapy - use of water as a medical treatment

hyperextension - active or passive force which takes the joint into extension, but beyond its normal physiological range.

hyperglycemia - high levels of glucose in the blood. 

hyperopia - farsightedness

hyperplasia, endometrial - excessive growth of cells in the endometrium.

hypertrophy - an increase in the size of tissue

hypogeusia - diminished sensitivity to taste 

hypomobility - a decrease in the normal range of joint movement.

hyposmia - diminished sensitivity to smell   

hypotension - abnormally low blood pressure

hypothalamus - small structure at the base of the brain that regulates many body functions, including appetite and body temperature.

hypoxia - depletion of oxygen in the cells and tissues

hysteroscope - visual examination of the canal of the cervix and the interior of the uterus using a viewing instrument (hysteroscope) inserted through the vagina.