Or center of gravity; The point in or near an object from which its total weight appears to originate and can be assumed to act. A symmetrical homogeneous object such as a sphere or cube has its center of mass at its physical center; a hollow shape (such as a cup) may have its center of mass in space inside the hollow.
For an object to be in stable equilibrium, a perpendicular line down through its center of mass must run within the boundaries of its base; if tilted until this line falls outside the base, the object becomes unstable and topples over.