Refer to enthalpy.

The amount of heat per unit mass that can be released when a substance undergoes a drop in temperature, a change in state or a chemical reaction.

The sum of the latent heat and sensible heat contained in a substance, above the heat contained at a specified temperature and pressure; expressed as Btu or calories per unit of volume or weight. See also CALORIFIC VALUE.

The amount of heat energy available to be released by the transformation or use of a specified physical unit of an energy form (e.g., a ton of coal, a barrel of oil, a kilowatthour of electricity, a cubic foot of natural gas, or a pound of steam). The amount of heat energy is commonly expressed in British thermal units (Btu). Note: Heat content of combustible energy forms can be expressed in terms of either gross heat content ( higher or upper heating value) or net heat content ( lower heating value), depending upon whether or not the available heat energy includes or excludes the energy used to vaporize water (contained in the original energy form or created during the combustion process). The Energy Information Administration typically uses gross heat content values.