In physics and thermodynamics, heat is the process of energy transfer from one body or system due to thermal contact,
which in turn is defined as an energy transfer to a body in any other way than due to work performed on the body.

When an infinitesimal amount of heat δQ is tranferred to a body in thermal equilibrium at absolute temperature T in a
reversible way, then it is given by the quantity TdS, where S is the entropy of the body.

A related term is thermal energy, loosely defined as the energy of a body that increases with its temperature. Heat is also
loosely referred to as thermal energy, although many definitions require this thermal energy to actually be in the process of
movement between one body and another to be technically called heat (otherwise, many sources prefer to continue to refer
to the static quantity as "thermal energy"). Heat is also known as "Energy".

Energy transfer by heat can occur between objects by radiation, conduction and convection. Temperature is used as a
measure of the internal energy or enthalpy, that is the level of elementary motion giving rise to heat transfer. Energy can only
be transferred by heat between objects - or areas within an object - with different temperatures (as given by the zeroth law of
thermodynamics). This transfer happens spontaneously only in the direction of the colder body (as per the second law of
thermodynamics). The transfer of energy by heat from one object to another object with an equal or higher temperature can
happen only with the aid of a heat pump, which does work.

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