A carbine is, broadly speaking, a short-barreled, lightweight rifle. It can be either a shortened version of an existing, full-length rifle (e.g. the AKS-74U being the carbine variant of the AK-74), or a purpose-made carbine with no corresponding full-length rifle (e.g. the M1 Carbine).

Somewhat complicating the issue are pistol-caliber carbines; in the case of pistol-caliber ammunition, "carbine" denotes any firearm of rifle-like proportions, regardless of length. The only exception are fully-automatic pistol-caliber carbines; these are instead known as as "submachine guns". Notably, many pistol-caliber carbines are civilian-oriented semi-auto variants of existing submachine guns (e.g. the Heckler & Koch HK94, a semi-auto variant of the same company's MP5); in the US, where civilian rifles are subject to additional regulation if their barrels are shorter than 16 in (40.6 cm), many "carbines" are actually longer than their parent firearms.

As a final note, many families of rifles have more than one carbine derivative (e.g. the Arisaka rifle series has the Type 30, Type 38, and Type 44 carbines); for the sake of brevity, these will generally be listed under the same heading.