Meaning of relativistic mass

rel'ativis'tic mass'

Pronunciation: [key]
— Physics. Physics.
  1. the mass of a body in motion relative to the observer: it is equal to the rest mass multiplied by a factor that is greater than 1 and that increases as the magnitude of the velocity increases.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.