The Collaborative International Dictionary of English v.0.48:

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.]
   [1913 Webster]
   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.
      [1913 Webster]

            The caliber of empty tubes.           --Reid.
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            A battery composed of three guns of small caliber.
                                                  --Prescott.
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   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.
         [1913 Webster]

   2. The diameter of round or cylindrical body, as of a bullet
      or column.
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   3. Fig.: Capacity or compass of mind. --Burke.
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   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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, calibre]
    2: diameter of a tube or gun barrel [syn: bore, gauge,
       caliber, calibre]