The Collaborative International Dictionary of English v.0.48:

Radius \Ra"di*us\ (r[=a]"d[i^]*[u^]s), n.; pl. L. Radii
   (r[=a]"d[i^]*[imac]); E. Radiuses
   (r[=a]"d[i^]*[u^]s*[e^]z). [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.
      [1913 Webster]

   2. (Anat.) The preaxial bone of the forearm, or brachium,
      corresponding to the tibia of the hind limb. See Illust.
      of Artiodactyla.
      [1913 Webster]

   Note: The radius is on the same side of the limb as the
         thumb, or pollex, and in man it is so articulated that
         its lower end is capable of partial rotation about the
         ulna.
         [1913 Webster]

   3. (Bot.) A ray, or outer floret, of the capitulum of such
      plants as the sunflower and the daisy. See Ray, 2.
      [1913 Webster]

   4. pl. (Zool.)
      (a) The barbs of a perfect feather.
      (b) Radiating organs, or color-markings, of the radiates.
          [1913 Webster]

   5. The movable limb of a sextant or other angular instrument.
      --Knight.
      [1913 Webster]

   Radius bar (Mach.), 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.
      [1913 Webster]