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Heat is a form of energy called thermal energy. It results from molecular activity in a substance. Energy cannot be created or destroyed, but can be converted from one form to another—a process that is continually going on.

The temperature of a substance depends on its average molecular activity, and will increase as the molecular activity increases and decrease with decreasing molecular activity. The quantity of heat that a substance contains depends on the sum, or total, of its molecular activity. Since the number of molecules varies in different substances, the total molecular activity must also vary. Hence, temperature can only indicate the relative degree of hotness or coldness of a substance, and not the quantity of heat it contains.

The standard unit of heat is the joule, derived from the heat produced by a standard electrical voltage applied to a standard resistance. Other units of heat in common use are the calorie and the British Thermal Unit (Btu). The calorie is arbitrarily defined as 4.1840 joules, and is very nearly equivalent to the amount of heat needed to raise the temperature of one gram of water from 14.5° to 15.5° C. A Btu is equal to 1055 joules or 252 calories. Specific heat is the quantity of heat needed to raise the temperature of a unit weight of a substance by 1°, while heat capacity is the quantity of heat required to raise the temperature of a unit volume by 1°.

A considerable amount of heat is often involved in the change of state of a substance. The amount of heat needed to change a liquid to a vapor, or released when the vapor is converted to a liquid, is the heat of vaporization. Heat released or absorbed in the changes of state of water is of major importance in weather processes and in the combustion of wildland fuels.