What is rest mass energy?

Question

What is rest mass energy?

Answer 1

It's the energy of a particle in its own proper rest frame, in which it is at rest.  It is proportional to the mass.  The constant of proportionality is the square of the speed of light.

Answer 2

In physics, mass–energy equivalence is the concept that any mass has an associated energy and vice versa. In special relativity this relationship is expressed using the mass–energy equivalence formula



where

E = energy,

m = mass,

c = the speed of light in a vacuum (celeritas),