Heat is energy on the move, or in transition. It is energy that is moved from one object to another when there is a temperature difference between them. Heat flows, energy is transferred, from the hotter object to the colder object. Usually (but not always) the temperature of the colder object increases while the temperature of the hotter object decreases.

1. Conduction: the movement of heat between objects at rest. Conductivity was first studied by Fourier and Biot in France in 1816, and the results published in 1822.

2. Convection: the movement of heat in liquids or gasses in a state of motion. Although Sir Isaac Newton proposed an equation to describe convection in 1701, this phenomenon was not understood fully until after 1880.

3. Radiation: the movement of heat across space with no atomic carrier. The governing principles of radiation were described by Gustav Robert Kirchhoff in 1859, and the relationship between heat radiated from a 'blackbody' and temperature was worked out by Stefan and Boltzmann and published in 1884.

As energy moves from one object to another, the temperature of the receiving object usually increases. The amount of energy needed to raise a defined amount of a substance by a defined temperature range is called the heat capacity, or the specific heat, of that substance. If, for example, heat is moved into a constant volume of gas, the amount of energy needed to cause a one-degree temperature rise is a standard value. But, if the heat is then added to the same gas, but in a cylinder that can expand, the amount of energy needed is greater than before.

In a non-expandable gas, all the added energy goes into raising the temperature of the gas, but when the gas is free to expand the added energy not only increase the temperature of the gas, it also has to provide the extra energy of expansion. Work is done when a force moves and object through a distance, so the expanding gas is doing work with the extra energy. If the cylinder of gas has a piston at one end, the expansion will move the piston, which in turn can do mechanical work (like in the cylinders of an automobile).

Sometimes an object that absorbs heat changes its physical state. For example a solid may turn into a liquid, or a liquid may turn into a gas or vapor.

Strictly speaking it is a mistake to say that there is 'heat in an object'. This is because the term 'heat' refers to the phenomenon of energy being moved from one object to another. Energy that is being stored in an object is not 'heat', but scientists often talk about 'latent heat' anyway.