The radial distance from a rotation axis at which the mass of an object could be concentrated without altering the moment of inertia of the body about that axis. If the mass M of the body were actually concentrated at a distance k from the axis, the moment of inertia about that axis would be Mk². If this quantity is equal to the actual moment of inertia I about the rotation axis, then k is the radius of gyration given by k = √I/M. The quantity k has dimensions of length and is measured in appropriate units of length such as meters.