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 A particle is Look up Particle in Wiktionary, the free dictionary In particle physics, a basic unit of matter or energy. ... In physics, an inertial frame of reference, or inertial frame for short (also descibed as absolute frame of reference), is a frame of reference in which the observers move without the influence of any accelerating or decelerating force. ...

E = m₀c²

where m₀ is the rest mass of the particle and c is the speed of light. The term mass in special relativity is used in a couple of different ways, occasionally leading to a great deal of confusion. ...

The concept of rest energy comes from special relativity theory where measurements of physical quantities may vary in different inertial reference frames. A simple introduction to this subject is provided in Special relativity for beginners Special relativity (SR) or the special theory of relativity is the physical theory published in 1905 by Albert Einstein. ...