Rest mass energy of the electron

Question

What do you mean by the rest mass energy of the electron?

Answer 1

According to the Einstein's Theory of Relativity, the mass of a body (state a particle) based on the energy and on the momentum (say velocity) with which the particle moves.

Now, we have a problem: Is there a mass value which every observer can relate to? Yes: it is the rest mass that is the mass you could measure in the frame of reference co-moving with the particle (in which the particle is still), that is the center of mass frame and which coincide with the minimum value measurable for each and every observers.