The law of conservation of mass and the analogous law of conservation of energy were finally generalized and unified into the principle of mass–energy equivalence, described by Albert Einstein's famous formula  E = mc^2. Special relativity also redefines the concept of mass and energy, which can be used interchangeably and are defined relative to the frame of reference. Several quantities had to be defined for consistency, such as the rest mass of a particle (mass in the rest frame of the particle) and the relativistic mass (in another frame). The latter term is usually less frequently used.
