center of mass
unique point where the weighted relative position of a distributed mass sums to zero
center of mass of the Earth
the center of mass of the planet Earth
centroid
mean ("average") position of all the points in the shape; mean position of all the points in all of the coordinate directions; point at which a cutout of the shape could be perfectly balanced on the tip of a pin
center of gravity
point around which the resultant torque due to gravity forces vanishes