Heat  —n. 1 condition of being hot. 2 physics form of energy arising from the motion of bodies' molecules. 3 hot weather. 4 warmth of feeling; anger or excitement. 5 (foll. By of) most intense part or period of activity (heat of battle). 6 (usu. Preliminary or trial) round in a race etc. —v. 1 make or become hot or warm. 2 inflame.  on heat (of mammals, esp. Females) sexually receptive. [old english]