What does the letter 'c' represent in Einstein's equation E=mc^2?

In Einstein's equation E=mc², the letter c represents the speed of light in vacuum.

The equation expresses mass–energy equivalence: energy and mass are two forms of the same physical quantity. Here, E is energy, m is rest mass, and c is the speed of light, approximately 299,792,458 metres per second. Because c is squared, even a small amount of mass corresponds to a very large amount of energy.

Einstein published the key result in 1905, although the relationship grew from the broader structure of special relativity. The equation does not mean that ordinary objects spontaneously release all their mass as energy. In nuclear reactions, however, a small mass difference can appear as a substantial energy release, as in nuclear fission and fusion.

A common mix-up is confusing c with the speed of light in any material. The symbol specifically refers to light's vacuum speed, which is also the universe's fundamental speed limit for information and causal influence.