H2-blockers  - medicines that reduce the amount of acid the stomach produces by blocking histamine2, which signals the stomach to make acid.

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.

Hepatologist - physician who specializes in liver diseases.

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 chorionic gonadotropin - hormone produced by the placenta during early pregnancy

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.