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Old English hætu, hæto "heat, warmth; fervor ardor," from Proto-Germanic *haiti- "heat" (cf. Old Saxon hittia, Old Norse hiti, Old Frisian hete, German hitze "heat," Gothic heito "fever"), from PIE *kaid-, from root *kai- "heat." The same root is the source of Old English hat "hot" and hæða "hot weather" (see hot).
Meaning "a single course in a race," especially a horse race, is from 1660s, perhaps from earlier figurative sense of "violent action; a single intense effort" (late 14c.), or meaning "run given to a horse to prepare for a race" (1570s). This later expanded to "division of a race or contest when there are too many contestants to run at once," the winners of each heat then competing in a final race. Meaning "sexual excitement in animals" is from 1768. Meaning "trouble with the police" attested by 1920. Heat wave "period of excessive hot weather" first attested 1890; earlier in reference to solar cycles.
A form of energy associated with the motion of atoms or molecules and capable of being transmitted through solid and fluid media by conduction, through fluid media by convection, and through empty space by radiation.
The sensation or perception of such energy as warmth or hotness.
An abnormally high bodily temperature, as from a fever.