What is radius of gyration?

Radius of gyration of a body about a given axis is the perpendicular distance of a point from the axis, where if whole mass of the body were concentrated, then the body shall have the same moment of inertia as it has with the actual distribution of mass.

This distance is represented by K. The momentum of inertia of a body of mass M and radius of gyration K is given by,

I=MK²