Hypothermia

n. condition in which the body temperature is much lower than normal

Hypothermia is a condition in which an organism's temperature drops below that required for normal metabolism and bodily functions. In warm-blooded animals, core body temperature is maintained near a constant level through biologic homeostasis. But when the body is exposed to cold its internal mechanisms may be unable to replenish the heat that is being lost to the organism's surroundings.
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Hypothermia, a potentially fatal condition, occurs when body temperature falls below 95°F. [more]Hypothermia - Community and Resources

Noun
1. subnormal body temperature
(antonym) hyperthermia, hyperthermy
(hypernym) physiological state, physiological condition

Abnormally low body temperature. Someone who falls asleep in a snowbank may become hypothermic. Hypothermia is intentionally produced to slow the metabolism during some types of surgery.