Definitions for "Center of mass"
The average position of the various mass elements of a body or system, weighted according to their distances from that center of mass; that point in an isolated system that moves with constant velocity, according to Newton's first law of motion.
The "balance point" of a group of objects, where the mass can be assumed to be for purposes of mechanical calculations; equivalent to the center of gravity.
The point at which the total mass of a body or system is assumed to be centered and upon which the sum of the external forces can act.