mass-energy equivalence

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

• noun The physical principle that a measured quantity of energy is equivalent to a measured quantity of mass. The equivalence is expressed by Einstein's equation E = mc², where E represents energy, m the equivalent mass, and c the speed of light.

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

• noun (physics) the principle that a measured quantity of mass is equivalent (according to relativity theory) to a measured quantity of energy