Caliber
The bore diameter of a firearm (and the cartridge it fires), usually given in inches or millimeters — e.g., .45 or 9mm.
Caliber refers to the internal diameter of the barrel's bore, and by extension the cartridge designed for it. It's expressed in hundredths/thousandths of an inch (.22, .308, .45) or in millimeters (9mm, 5.56mm). Confusingly, caliber names aren't always exact bore measurements and often include a cartridge name (.357 Magnum vs. .38 Special share a bullet diameter but differ in case).
Caliber influences recoil, capacity, cost, and terminal performance, but the cartridge as a whole — case, powder, and bullet — defines real-world behavior. For smoothbore shotguns, bore size is described by 'gauge' instead of caliber (with .410 being the exception, named like a caliber).