The total amount of heat in a body, process, or reaction. The SI unit is joules. The calorie is also still used, which is the amount of heat required to raise the temperature of one gram of water by 1^ºC. The definition is not very precise since the specific heat capacity of water varies with temperature. In the f.p.s. engineering unit system, the British thermal unit is defined as the heat required to raise the temperature of one pound of water through one degree Fahrenheit. The therm is 10⁵ Btu.