Centre of mass

In physics, the center of mass of a system of particles is a specific point at which, for many purposes, the system's mass behaves as if it were concentrated. The center of mass is a function only of the positions and masses of the particles that comprise the system. In the case of a rigid body, the position of its center of mass is fixed in relation to the object (but not necessarily in contact with it). In the case of a loose distribution of masses in free space, such as, say, shot from a shotgun, the position of the center of mass is a point in space among them that may not correspond to the position of any individual mass. In the context of an entirely uniform gravitational field, the center of mass is often called the center of gravity — the point where gravity can be said to act.
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Noun
1. point representing the mean position of the matter in a body
(synonym) center of mass
(hypernym) center, centre, midpoint
(hyponym) center of buoyancy, centre of buoyancy, center of immersion, centre of immersion

The point at which a system of masses would balance if placed on a pivot. For example, in a binary star system, the centre of mass would be nearer to the more massive star than to the less massive component. In the case of a single mass, the centre of gravity, for the sake of simplicity of calculations, is the point at which it can be assumed all the mass is concentrated.