Mass in special relativity incorporates the general understandings from the concept of mass–energy equivalence. The word "mass" is given two meanings in special relativity: one ("rest mass" or "invariant mass") is an invariant quantity which is the same for all observers in all reference frames; the other ("relativistic mass") is dependent on the velocity of the observer. Roche states that about 60% of modern authors just use rest mass and avoid relativistic mass.