Definition of
moment of inertia. : Is analogous to the mass of a moving object. Moment of Inertia or mass is the measure of the tendency of the object to resist a change in motion.
Radius of Gyration of a body about an axis of rotation is defined as the radial distance to a point which would have a Moment of Inertia the same as the body's actual distribution of mass, if the total mass of the body were concentrated
Ref:
https://en.wikipedia.org/wiki/Radius_of_gyration