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 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. COM can refer to: .com, Internet top-level domain COM file (short for command), a simple MS-DOS executable format Hardware Communication Ports (COM:), prefix in Microsoft Windows and MS-DOS operating system Component Object Model, popularized by Microsoft COM (manga magazine), an alternative manga magazine founded by Osamu Tezuka... In physics, an inertial frame of reference, or inertial frame for short (also descibed as absolute frame of reference), is a frame of reference in which the observers move without the influence of any accelerating or decelerating force. ... In physics, the center of mass (or centre of mass) of a system of particles is a specific point at which, for many purposes, the systems mass behaves as if it were concentrated. ...

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. The term mass in special relativity is used in a couple of different ways, occasionally leading to a great deal of confusion. ... In particle physics, the mathematical combination of a particles energy and its momentum to give a value for the mass of the particle at rest. ...