The process of energy transfer from one body or system to another as a result of a difference in temperature. The energy in the body or system before or after transfer is also sometimes called heat but this leads to confusion, especially in thermodynamics.

A body in equilibrium with its surroundings contains energy (the kinetic and potential energies of its atoms and molecules) but this is called internal energy, U, rather than heat. When such a body changes its temperature or phase there is a change in internal energy, Δ‎U, which (according to the first law of thermodynamics) is given by Δ‎U=Q − W, where Q is the heat absorbed by the body from the surroundings and W is the work done simultaneously on the surroundings. To use the word ‘heat’ for both U and Q is clearly confusing. Note also that certain physical quantities are described as heat of atomization, heat of combustion, etc. What is usually used is a standard molar enthalpy change for the process under consideration. The units are kJ mol⁻¹; a negative value indicates that energy is liberated. See also heat capacity; heat transfer; latent heat.