The equation "E=mc²" is a formula developed by physicist Albert Einstein as part of his theory of relativity. It expresses the relationship between energy (E) and mass (m), stating that energy is equal to mass multiplied by the speed of light (c) squared. This means that a small amount of mass can be converted into a large amount of energy, highlighting the potential energy stored in matter. In practical terms, this principle explains phenomena such as nuclear reactions, where tiny amounts of mass are transformed into significant energy. The equation has profound implications in fields like nuclear physics and cosmology, influencing our understanding of the universe.