Hypothermia (n) : subnormal temperature of the body, defined as temperature under 36.6°C
New Latin; hypo-, low, and therm-, heat.
Hypothermia can be accidental, from exposure to cold, or therapeutic. Therapeutic hypothermia has been shown to produce better neurologic outcomes post-cardiac arrest, for example. Since hypothermia slows metabolism, neurons are better preserved. It is also used in certain cases after head traumas or hemorrhages.
Bonus words: normothermia is normal body temperature, and hyperthermia is a fever.