The Collaborative International Dictionary of English v.0.48:

Relativistic mass equation \Rel`a*tiv*ist"ic mass e*qua"tion\,
   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/root1 - v^{2/c^2 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]

   Note: 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]