invariant (rest) mass

The invariant mass is the genuine, built-in mass of an object — the number every observer agrees on, no matter how fast they are moving relative to it. It is computed from the object's energy and momentum together by m = sqrt(E^2 - (p c)^2)/c^2. Although different observers disagree about E and about p separately, this particular combination always comes out the same, which is why physicists call it invariant.

This is the modern meaning of the bare word mass. Crucially, it does not change with speed: an electron has the same invariant mass whether it drifts slowly through a wire or screams around a collider. What grows with speed is the object's energy and momentum, not its mass. Treating m as a fixed property and putting all the speed dependence into the explicit gamma factor keeps the physics clean.

For a system of several particles, the invariant mass is not just the sum of the individual masses — it also captures the energy of their relative motion and binding. This is how an unstable particle is reconstructed: detectors measure the energies and momenta of its decay fragments, and the invariant mass of the bunch reveals the mass of the parent that produced them.