Center of gravity
From AMS Glossary
center of gravity
The point at which the resultant gravitational force acts upon an object.
The center of gravity is not necessarily inside the object. For example, the center of gravity of a ring is at the center of symmetry. If the geometry of the object does not change with time, the center of gravity will remain unchanged in relation to the object. The mathematical definition of center of gravity is
where vector x describes the position of the object in a given coordinate system, ρ = ρ(x) is the density, and V is the volume of the object.