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.
      [1913 Webster]

            A battery composed of three guns of small caliber.
                                                  --Prescott.
      [1913 Webster]

   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.
      [1913 Webster]

   3. Fig.: Capacity or compass of mind. --Burke.
      [1913 Webster]

   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):

caliber
    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]

Moby Thesaurus II by Grady Ward, 1.0:

150 Moby Thesaurus words for "caliber":
   IQ, ability, ableness, adequacy, amount, amplitude, apprehension,
   area, bigness, body, bore, breadth, bulk, bump, capability,
   capableness, capacity, class, compass, competence, comprehension,
   conception, coverage, cut, deductive power, degree, depth,
   diameter, dimension, dimensions, dower, dowry, efficacy,
   efficiency, endowment, equipment, esemplastic power, expanse,
   expansion, extension, extent, facility, faculty, fitness, flair,
   force, forte, gauge, genius, gift, girth, grade, greatness, height,
   ideation, instinct, integrative power, intellect,
   intellectual grasp, intellectual power, intellectualism,
   intellectuality, intelligence, intelligence quotient, interval,
   knowledge, largeness, leap, length, level, long suit, magnitude,
   makings, mark, mass, measure, measurement, mental age,
   mental capacity, mental grasp, mental ratio, mentality, merit,
   metier, mother wit, native wit, natural endowment, natural gift,
   notch, nuance, parts, pas, peg, period, pitch, plane, plateau,
   point, potential, power, power of mind, powers, proficiency,
   proportion, proportions, qualification, radius, range, ratio,
   rationality, reach, reasoning power, remove, round, rung, sanity,
   scale, scope, scope of mind, semidiameter, sense, shade, shadow,
   size, space, speciality, spread, stair, standard, stature, step,
   stint, strong flair, strong point, sufficiency, susceptibility,
   talent, talents, the goods, the stuff, thinking power, tread,
   understanding, value, virtue, volume, what it takes, width, wit,
   worth