In the equation E=mc², the "E" represents energy. This famous equation, formulated by physicist Albert Einstein, is a fundamental principle of his theory of relativity. It describes the equivalence of mass (m) and energy, indicating that mass can be converted into energy and vice versa. The "c" in the equation stands for the speed of light in a vacuum, squared, emphasizing the tremendous amount of energy contained within even small amounts of mass due to its large multiplier effect. E=mc² has had profound implications in physics, influencing everything from atomic physics to cosmology.