the distance from an axis at which the mass of a body may be assumed to be concentrated and at which the moment of inertia will be equal to the moment of inertia of the actual mass about the axis, equal to the square root of the quotient of the moment of inertia and the mass.
British Dictionary definitions for radius-gyration
radius of gyration
a length that represents the distance in a rotating system between the point about which it is rotating and the point to or from which a transfer of energy has the maximum effect. Symbol: k or r. In a system with a moment of inertia I and mass m, k² = I/m