The term 'Unit of Heat' can be defined by three crossword solutions: CALORIE (7 letters), therm (5 letters), and COP (3 letters). A CALORIE is a unit of energy used to measure the amount of heat required to raise the temperature of one gram of water by one degree Celsius. A therm is a unit of energy equal to 100, 000 British thermal units (BTUs). A COP, or coefficient of performance, is a measure of the efficiency of a heat pump, which is a device that transfers heat from one place to another.