Mass–Energy Equivalence

A body at rest has an energy content proportional to its inertial mass. The modern statement $E=mc^2$ says that rest energy equals mass times the square of the speed of light.

Einstein did not write $E=mc^2$ in the 1905 paper. He concluded that if a body gives off energy $L$ as radiation, its mass diminishes by $L/c^2$. The familiar rest-energy form is the same claim in modern notation.

In a frame where the system moves, the same algebra relates relativistic energy and relativistic mass. The seed entry treats the rest-energy statement.