1. A term used to designate the specific cartridge(s) for which a firearm is chambered. 2. Firearms: The approximate diameter of the circle formed by the tops of the lands of a rifled barrel, often expressed in hundredths of an inch (“.38 Caliber”) or millimeters (“7mm Caliber). 3. Ammunition: A numerical term included in a cartridge name to indicate a rough approximation of the bullet diameter.