[Rest Mass Energy] The term refers to the concept of mass-energy equivalence. Einstein proposed that the equivalence of mass and energy is a general principle.
E=mc²
E = energy,
m = mass,
c = the speed of light in a vacuum.
c² is the conversion factor required to convert from units of mass to units of energy.
The rest energy of an electron or a positron for example amounts to 0.511 MeV.

See also Photon, Photon Energy.