The medical term for an abnormally low body temperature caused by cold exposure is hypothermia.
Hypothermia occurs when the body loses heat faster than it can produce it, lowering core temperature below 35 °C (95 °F). Cold air and immersion in cold water are classic causes, but illness, blood loss, low blood sugar, alcohol, and advanced age can also make temperature regulation fail.
Early hypothermia commonly causes shivering, rapid breathing, and confusion. As the condition worsens, shivering may stop, coordination and judgment deteriorate, and the heart can become dangerously unstable. Frostbite is different: it is a local freezing injury to skin and other tissues, while hypothermia affects the body’s core temperature.
The word combines the Greek roots hypo-, meaning “under,” and thermē, meaning “heat.” Hyperthermia is the opposite problem: an abnormally high body temperature caused by excessive heat or failed heat regulation, rather than a cold-induced loss of body heat.