A shotgun is a long-barreled firearm designed to shoot a straight-walled cartridge known as a shotshell, which usually discharges numerous small pellet-like spherical sub-projectiles called shot, or sometimes a single solid projectile called a slug. Shotguns are most commonly smoothbore firearms, meaning that their gun barrels have no rifling on the inner wall, but rifled barrels for shooting slugs (slug barrels) are also available.

Shotguns come in a wide variety of calibers and gauges ranging from 5.5 mm (.22 inch) to up to 5 cm (2.0 in), though the 12-gauge (18.53 mm or 0.729 in) and 20-gauge (15.63 mm or 0.615 in) bores are by far the most common. Almost all are breech loading, and can be single-barreled, double-barreled, or in the form of a combination gun. Like rifles, shotguns also come in a range of different action types, both single-shot and repeating.