According to the Einstein's Theory of Relativity, the mass

of a body (say a particle) depends on the energy and on the

momentum (say the velocity) with which the particle moves.

So, we have a problem: is there a mass value that every

observers can relate to? Yes: is the rest mass, that is the

mass you could measure in a frame of reference co-moving

with the particle (in which the particle is still), that is

the center-of-mass frame and that coincide with the minimum

value measurable for every observers.