Source : Webster's Revised Unabridged Dictionary (1913)

Radius \Ra"di*us\, n.; pl. L. {Radii}; E. {Radiuses}. [L., a
   staff, rod, spoke of a wheel, radius, ray. See {Ray} a
   divergent line.]
   1. (Geom.) A right line drawn or extending from the center of
      a circle to the periphery; the semidiameter of a circle or
      sphere.

   2. (Anat.) The preaxial bone of the forearm, or brachium,
      corresponding to the tibia of the hind limb. See Illust.
      of {Artiodactyla}.

   Note: The radius is on the same side of the limb as the
         thumb, or pollex, and in man it so articulated that its
         lower end is capable of partial rotation about the
         ulna.

   3. (Bot.) A ray, or outer floret, of the capitulum of such
      plants as the sunflower and the daisy. See {Ray}, 2.

   4. pl. (Zo["o]l.)
      (a) The barbs of a perfect feather.
      (b) Radiating organs, or color-markings, of the radiates.

   5. The movable limb of a sextant or other angular instrument.
      --Knight.

   {Radius bar} (Math.), a bar pivoted at one end, about which
      it swings, and having its other end attached to a piece
      which it causes to move in a circular arc.

   {Radius of curvature}. See under {Curvature}.

Curvature \Cur"va*ture\ (k?r"v?-t?r; 135), n. [L. curvatura. See
   {Curvate}.]
   1. The act of curving, or the state of being bent or curved;
      a curving or bending, normal or abnormal, as of a line or
      surface from a rectilinear direction; a bend; a curve.
      --Cowper.

            The elegant curvature of their fronds. --Darwin.

   2. (Math.) The amount of degree of bending of a mathematical
      curve, or the tendency at any point to depart from a
      tangent drawn to the curve at that point.

   {Aberrancy of curvature} (Geom.), the deviation of a curve
      from a circular form.

   {Absolute curvature}. See under {Absolute}.

   {Angle of curvature} (Geom.), one that expresses the amount
      of curvature of a curve.

   {Chord of curvature}. See under {Chord}.

   {Circle of curvature}. See {Osculating circle of a curve},
      under {Circle}.

   {Curvature of the spine} (Med.), an abnormal curving of the
      spine, especially in a lateral direction.

   {Radius of curvature}, the radius of the circle of curvature,
      or osculatory circle, at any point of a curve.

Source : WordNet®

radius of curvature
     n : the radius of the circle of curvature; the absolute value of
         the reciprocal of the curvature of a curve at a given
         point