Proper Energy

The proper energy of a body or a particle is the energy of the body as measured in the proper frame of reference—that is, in the reference frame in which the body is at rest. The proper energy is the same as the rest energy and is equal to E₀ = mc², where m is the rest mass of the body and c is the speed of light in a vacuum. (SeeRELATIVITY, THEORY OF.)