conservation of mass-energy The principle that in any system of interacting bodies the total mass plus total energy is constant, where energy and mass are to be considered interconvertible according to the equation E = mc 2. The principle is a result of the theory of special relativity and is a generalization of the classical laws of conservation of energy and conservation of mass. These state that in a closed system energy (or matter) cannot appear or disappear spontaneously and cannot be lost or gained. Energy can only be transformed into different forms but the total energy must remain constant. According to relativity theory the transfer of energy by any process entails the transfer of mass, so that conservation of energy ensures conservation of mass.