CENTER OF MASS FRAME

The 'center of mass frame' (also called the 'center of momentum frame', 'CM frame', or 'COM frame') is defined as being the particular inertial frame in which the center of mass of a system of interest is at rest (has zero velocity). In this special inertial frame, the total linear momentum of the system is zero, and the total energy of the system is the minimal energy as seen from all possible inertial reference frames.
In this COM frame, the total energy of the system is the "rest energy", and this quantity (when divided by the factor c²) therefore gives the rest mass or invariant mass of the system.