Carbine is a firearm, similar to but shorter than an ordinary rifle or musket in barrel and stock. The carbine was originally developed to be used by cavalry soldiers, for whom a full-length rifle was too heavy and cumbersome to be fired from horseback. Carbines have also been issued to secondary troops, such as cooks, technicians etc., for whom a lighter weapon is sufficient and desirable. Many military rifles have been manufactured in both full-length and carbine versions. After WWII the assault rifle, which is basically a cross-breed of the carbine and a submachine gun, has become the standard infantry weapon, and carbine as a category of military firearms has virtually vanished.
Today the term 'carbine' is used on a variety of firearms, basically anything between a pistol and a rifle. Pistols with a shoulder stock, submachine guns or assault rifles with only single fire capability and small rifles designed to fire pistol cartridges can be seen to bear the title. The Colt M4 is the main carbine of the U.S. Army.