Kilogram
The weight of matter depends on the strength of gravity. The mass of matter does not. The mass of an object is the same everywhere. Matter has invariant mass assuming it is not traveling at a relativistic speed with respect to an observer. According to Einstein’s theory of special relativity, the relativistic mass (apparent mass with respect to an observer) of an object or particle with rest mass m0 increases with its speed as M=γm0 (where γ is the Lorentz factor). This effect is vanishingly small at everyday speeds, which are by orders of magnitude less than the speed of light, but becomes noticeable at very high speeds. For example, traveling at just 10% the speed of light with respect to an observer—exceedingly fast compared to everyday speeds (about )—increases an object’s relativistic mass just over 0.5%.