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The 'rest energy' $E$ or rest mass-energy of a particle is its energy when it is at rest relative to a given inertial reference frame. It is defined by
:$E=m\_0\; c^2$,
where $m\_0$ is the rest mass of the particle and $c$ is the speed of light in a vacuum.
The equivalence of mass and energy can be derived from the Special Theory of Relativity. The mass of a given body is proportional to its kinetic energy, viz.
,
with the implication that energy and mass are equivalent. Therefore, a particle which remains at rest with respect to a given inertial frame has a certain amount of energy if it has a rest mass. Like other forms of energy, rest energy may be converted to other forms of energy as in nuclear fission.