Source : Webster's Revised Unabridged Dictionary (1913)

Radii \Ra"di*i\, n.,
   pl. of {Radius}.

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}.

Source : WordNet®

radius
     n 1: the length of a line segment between the center and
          circumference of a circle or sphere [syn: {r}]
     2: a straight line from the center to the perimeter of a circle
        (or from the center to the surface of a sphere)
     3: a circular region whose area is indicated by the length of
        its radius; "they located it within a radius of 2 miles"
     4: the outer and slightly shorter of the two bones of the human
        forearm
     5: support consisting of a radial member of a wheel joining the
        hub to the rim [syn: {spoke}]
     [also: {radii} (pl)]

radii
     See {radius}