rest energy

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The energy equivalent of the rest mass of a body, equal to the rest mass multiplied by the speed of light squared.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics The energy of the mass component of a body at rest; equal to the product its rest mass and the square of the speed of light

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the energy equivalent to the mass of a particle at rest in an inertial frame of reference; equal to the rest mass times the square of the speed of light