A position relative to an object or system of objects is defined as the center of mass.
It is the weighted average of all the system's parts' positions.
The centroid of simple rigid objects with uniform density is the location of the center of mass.
The center of mass is the location in the body where all of the mass is concentrated. Because the body is balanced at this point, there is no net torque (force) acting on it, and it remains in equilibrium.