For years, and even recently, the term "rest mass" has been used. Well, I hope that everyone knows that the magnitude of a 3-vector is invariant under Galilean transformations between coordinate systems (or inertial frames). Under Poincare transformations between inertial frames (coordinate systems) the magnitude of a
4-vectpr is invariant. This means that the value does not change from one system to the another. The magnitude of the 4-momentum is the mass. What part of "invariant" do some physicists not get?