mass-energy equivalence

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. 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.

WordNet 3.0

1. n. (physics) the principle that a measured quantity of mass is equivalent (according to relativity theory) to a measured quantity of energy