In the equation E=mc², the 'E' represents energy. This famous equation, formulated by Albert Einstein as part of his theory of relativity, illustrates the relationship between energy (E), mass (m), and the speed of light (c), which is squared (c²). The equation conveys that mass can be converted into energy and vice versa, highlighting one of the key concepts in modern physics. This insight has had profound implications, influencing not only theoretical physics but also practical applications such as nuclear energy. The equation encapsulates the idea that small amounts of mass can yield tremendous quantities of energy.