is the measure of the available energy content of a fuel, under controlled conditions specified by test method ASTM-D-240 or D-2382. Heat of combustion is determined by burning a small quantity of a fuel in an oxygen bomb calorimeter and measuring the heat absorbed by a specified quantity of water within the calorimeter. Heat of combustion is expressed either as calories per gram or British thermal units per pound. Also called thermal value, heating value, calorific value.