the point at which the entire mass of a body may be considered concentrated for some purposes; formally, the point such that the first moment of a physical or geometric object about every line through the point is zero.Compare moment(def 8).

A point in space determined by a distribution of mass (such as a solid object, a collection of objects, or a gas), such that a uniform force acting on the whole distribution acts as if the distribution were located at just that point. For example, the gravitational force on Earth attracts small objects as if it were pulling them from their center of mass (in this case, the center of mass is also called the center of gravity). Two stars in orbit around each other revolve around their collective center of mass. See also barycenter.