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.]
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.
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
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.