1. of a mass : the ratio of the torque applied to a rigid body free to rotate about a given axis to the angular acceleration thus produced about that axis and equal to the sum of the products of each element of mass by the square of its distance from the given axis — called also rotational inertia
2. of an area : the sum of the products of each element of a plane area by the square of its distance from a given axis in the plane of the area