The rest energy $E$ of a particle is its energy when it is not moving 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.

The concept of rest energy comes from special relativity theory where measurements of physical quantities may vary in different inertial reference frames.

Relativity | Energía en reposo