Mass as a Historical Alias for Rest Energy

This paper defends the thesis that mass is not an independent fundamental physical quantity but a historical alias for rest energy. The usual modern clarification rejects relativistic mass while retaining invariant mass, but that clarification remains incomplete. In relativistic dynamics, inertia is governed fundamentally by total energy and momentum; in gravitation, the more general source is the energy--momentum tensor; and the quantity traditionally called invariant mass is simply rest energy expressed in units of 1/c^2. On this reading, Einstein's deeper contribution was not merely to connect two primitive quantities through E₀=mc^2, but to reveal that the invariant quantity historically called mass is just rest energy in another unit system. The paper argues that retaining mass as a primitive concept sustains conceptual duplication, pedagogical confusion, and avoidable obscurity in the interpretation of inertia, gravitation, the weak equivalence principle, and the Brout--Englert--Higgs mechanism. It concludes that mass may remain a practical abbreviation, but should no longer be treated in foundational discourse as an autonomous primitive.