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}.