Mass in special relativity incorporates the general understandings from the concept of
mass–energy equivalence. Added to this concept is an additional complication resulting from the fact that
mass is defined in two different ways in
special relativity: one way defines mass ("rest mass" or "invariant mass") as an invariant quantity which is the same for all observers in all reference frames; in the other definition, the measure of mass ("relativistic mass") is dependent on the velocity of the observer.