relativistic mass

Relativistic mass is an older way of talking about relativity in which an object's mass is said to grow with speed, given by gamma m, where m is the ordinary rest mass and gamma = 1/sqrt(1 - v^2/c^2). In this language a moving object becomes 'heavier', and its rising momentum and energy are blamed on this growing mass rather than on the gamma factor itself. It was a popular teaching shortcut for much of the twentieth century.

The appeal is that it lets you keep Newton-looking formulas like p = (relativistic mass) times v and E = (relativistic mass) times c^2. The trouble is that it is misleading: a moving object does not actually become harder to deflect sideways and to speed up by the same factor, so a single 'mass' cannot honestly stand in for inertia in every direction. It also tempts people into the false idea that fast objects could collapse into black holes simply by moving.

Modern physics therefore discourages the term. The preferred picture uses one fixed, frame-independent mass — the invariant mass — and writes the speed dependence out openly as explicit gamma factors in the energy and momentum. The word mass, used plainly, now means the invariant mass and nothing else.