Mass–energy equivalence is a fundamental principle of physics stating that mass and energy are interchangeable, meaning that mass can be converted into energy, and vice versa. This relationship is quantified by relating the amount of energy contained within a given mass to the square of the speed of light. The mathematical formulation demonstrates that even a small amount of mass corresponds to a vastly large amount of intrinsic energy. This principle is crucial for understanding nuclear reactions, where changes in atomic structure result in the release of immense amounts of energy.