mass–energy equivalence

the principle that energy and mass, even though existing as different forms, are interchangeable forms of the same physical quantity, as represented by Einstein's signature equation, E = mc², in which E is energy, m is mass, and c is the speed of light.

An equation derived from Einstein's theory of Special Relativity expressing the relationship between the mass and energy of objects with mass. The equation is E = mc ², where E is the energy of the object in joules, m is its relativistic mass in kilograms, and c is the speed of light (approximately 3 × 10 ⁸ meters per second). Mass-energy equivalence entails that the total mass of a system may change, although the total energy and momentum remain constant; for example, the collision of an electron and a proton annihilates the mass of both particles, but creates energy in the form of photons. The discovery of mass-energy equivalence was essential to the development of theories of atomic fission and fusion reactions.