Radius of gyration is 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.
[img source: Limbrunner and Spiegel. Applied Statics and Strength of Materials. 2nd ed. Upper Saddle River, NJ: Prentice Hall, 1991.]
