Source : Webster's Revised Unabridged Dictionary (1913)

Caliber \Cal"i*ber\, Calibre \Cal"ibre\, n. [F. calibre, perh.
   fr. L. qualibra of what pound, of what weight; hence, of what
   size, applied first to a ball or bullet; cf. also Ar.
   q[=a]lib model, mold. Cf. {Calipers}, {Calivere}.]
   1. (Gunnery) The diameter of the bore, as a cannon or other
      firearm, or of any tube; or the weight or size of the
      projectile which a firearm will carry; as, an 8 inch gun,
      a 12-pounder, a 44 caliber.

            The caliber of empty tubes.           --Reid.

            A battery composed of three guns of small caliber.
                                                  --Prescott.

   Note: The caliber of firearms is expressed in various ways.
         Cannon are often designated by the weight of a solid
         spherical shot that will fit the bore; as, a
         12-pounder; pieces of ordnance that project shell or
         hollow shot are designated by the diameter of their
         bore; as, a 12 inch mortar or a 14 inch shell gun;
         small arms are designated by hundredths of an inch
         expressed decimally; as, a rifle of .44 inch caliber.

   2. The diameter of round or cylindrical body, as of a bullet
      or column.

   3. Fig.: Capacity or compass of mind. --Burke.

   {Caliber compasses}. See {Calipers}.

   {Caliber rule}, a gunner's calipers, an instrument having two
      scales arranged to determine a ball's weight from its
      diameter, and conversely.

   {A ship's caliber}, the weight of her armament.

Source : WordNet®

calibre
     n 1: a degree or grade of excellence or worth; "the quality of
          students has risen"; "an executive of low caliber" [syn:
           {quality}, {caliber}]
     2: diameter of a tube or gun barrel [syn: {bore}, {gauge}, {caliber}]