Nuclear Physics Question:

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.