a. 1. of, pertaining to, or in conformity with the theory of relativity; as, the relativistic increase of mass with velocity. [ PJC ]

2. moving at a speed sufficiently high that the changes of mass or time dilation predicted by the theory of relativity may be observed; moving at a significant fraction of the speed of light; as, relativistic electrons.br/ [ PJC ]

n. (physics) an equation expressing the mass of an object as a function of its velocity: as the velocity v of an object increases, its mass m increases from its rest mass m₀ according to the equation: m = m₀/1 - v2/c2 where c is the speed of light in a vacuum. If the speed of an object were to become equal to that of light, it can be seen that the mass would be infinite, from which it follows that nothing can accelerate up to or beyond the speed of light. [ PJC ]

☞ The theoretical possibility that there are particles which always move faster than light cannot at present be disproved and is not denied by this equation; in such a case, the relativistic mass equation asserts that such particles can never decelerate to or below the speed of light. Such hypothetical particles are referred to as
tachyons. [ PJC ]