Heat is the energy exchanged when a difference in temperature exists between two regions. That is to say, changes in temperatures are produced by the addition or subtraction of heat from a substance.

In general, the more heat that is added to a particular body of matter, the more the temperature of that body rises. When heat is added to or removed from a physical system, the accompanying thermal changes may include a change in length and volume, and/or changes in physical states such as melting, evaporation, etc. The addition or removal of heat from a material may destroy the magnetic, superconductive, ferroelectric, shear, and/or cohesive properties of that material, and it may profoundly affect the mechanical properties of solids, fluidity of liquids, the electrical conductivity of metals, and the thermal conductivity of solids.

During the eighteenth century, heat was believed to be a fluid, called the caloric, that filled the space between the fundamental particles of matter. Once it was recognized that friction produces heat and that heat can be dissipated when mechanical work is performed, heat was viewed as another form of energy.

When physical changes in a body accompany the addition or removal of heat from that body, finite quantities of heat may be absorbed over an infinitesimal temperature range (at constant pressure), resulting in a change of phase. The heat absorbed at constant pressure per mole when a solid melts is referred to as the heat of melting. The heat of vaporization is the heat absorbed at constant pressure per mole when a liquid vaporizes. And the heat of sublimation is the heat absorbed at constant pressure per mole when the solid vaporizes.