Webster's New World Medical Dictionary

hydroxyapatite crystal disease
Inflammation
caused by hydroxyapatite crystals. Hydroxyapatite
molecules can group together (crystallize) to form
microscopic clumps. If the tiny crystals of hydrox-
yapatite are deposited by mistake in or around
joints, they may cause inflammation of the joints and
nearby tissues, such as tendons and ligaments.
Hydroxyapatite crystal disease is sometimes the
cause of rotator cuff problems in the shoulder.
hygiene
The science of preventive medicine and
the preservation of health. Also commonly used as a
euphemism for cleanliness and proper sanitation.
hymen
A thin membrane that may completely or
partially cover the vaginal opening before first sex-
ual intercourse but that usually disappears before
puberty.
hyoglossus
The muscle that permits the tongue
to be held on the floor of the mouth.
hyper-
Prefix meaning high, beyond, excessive,
or above normal, as in hyperglycemia (high sugar in
the blood) and hypercalcemia (high calcium in the
blood). The opposite of hyper- is hypo-.
hyperactivity
A higher-than-normal level of activ-
ity. An organ can be described as hyperactive if it is
more active than is usual. Behavior can also be
described as hyperactive. See also 
attention deficit
hyperactivity disorder.
hyperadrenocorticism
See 
Cushing’s syndrome.
hyperaldosteronism
See 
aldosteronism.
hyperbaric oxygen chamber
A pressurized
chamber in which a patient receives pure oxygen,
either directly or through a mask, tent, or tube. The
oxygen is delivered at high pressure, more than 1.4
times normal atmospheric pressure. See also
hyperbaric oxygen therapy.
hyperbaric oxygen therapy
The use of a hyper-
baric oxygen (HBO) chamber to treat any of a num-
ber of conditions, notably carbon monoxide
poisoning, decompression sickness (“the bends”),
smoke inhalation, and gas gangrene. HBO therapy
also helps to heal skin grafts, major burn injuries,
and it is sometimes used in cases of antibiotic-
resistant or severe infection. The patient is enclosed
in the chamber and receives appropriately pressur-
ized pure oxygen for a specified length of time.