What does 'c' represent in Einstein's mass-energy equivalence equation?

In Einstein’s mass–energy equivalence equation, c represents the speed of light in a vacuum.

The equation E = mc² links an object’s rest energy E with its mass m. Because c is squared, even a tiny amount of mass corresponds to an enormous quantity of energy—about 9 × 10¹⁶ joules per kilogram.

Albert Einstein developed the relationship through special relativity. His 1905 paper on whether a body’s inertia depends on its energy content showed that an object loses mass when it emits energy as radiation. The compact form E = mc² became the famous shorthand for that deeper result.

A common mix-up is confusing c with the gravitational constant G, which appears in Newtonian gravity, or with the charge of the electron. The c in this equation is not the speed of light through glass or water: those speeds are lower than the vacuum value. The same constant also appears in relativity’s energy–momentum relation and sets the ultimate speed limit for matter and information.