1. (n.) 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.

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

3. (n.) Fig.: Capacity or compass of mind.