What constant is denoted by 'c' in Einstein's equation E=mc^2?

In Einstein’s equation E=mc², c denotes the speed of light in a vacuum. Its exact value is 299,792,458 metres per second, and the squared term makes the mass–energy relationship extraordinarily powerful.

The equation expresses mass–energy equivalence: mass can be understood as a concentrated form of energy. Because c is such a large number, even a small amount of mass corresponds to a very large amount of energy. The equation is associated with Einstein’s 1905 work on special relativity, although the compact form E=mc² became widely popular later.

The c is not the speed of light through glass, water, or another material. It specifically refers to the invariant speed of light in empty space. Light travels more slowly through matter, while c remains a fundamental constant in relativity.

Another common mix-up is to treat c as a variable that changes with the observer’s motion. In special relativity, the speed of light in a vacuum is the same for all inertial observers. The symbol may have connections to words meaning “constant” or “celerity,” but in this equation its physical meaning is the speed of light in vacuum.