The internal energy of a substance, associated with the positions and motions of its component molecules, atoms, and ions.

The average kinetic energy of the molecules or atoms, which is due to their motions, is measured by the Temperature of the substance. The potential energy is associated with the state, or phase, of the substance (states of matter).

Heat energy is commonly expressed in Calories, British Thermal Units, or Joules.

Heat may be transferred from one substance to another by three means Conduction, Convection, and Radiation.