In modern terms: a firearm falling in size between that of a submachinegun and that of a full-size rifle. Modern examples include the M4 Carbine- a shortened variant of the M-16- and AKSU- a shortened variant of the AK-47. Earlier examples include the American M1 Carbine and M2 Carbine.
Carbines are typically designed for use in close-quarters combat, specifically within situations wherein the bulk of a full-size rifle is prohibitive. As such, they commonly feature low-power optical or iron sights, fully-automatic fire, and light weight, enabling a soldier to rapidly track and kill his enemy in conditions where a split-second delay can be fatal.
Carbines also specialize in medium-range combat, which is commonly found during urban engagements. With accuracy and power greater then a submachinegun, they can often win out in firefights over ranges from 25-75 meters.
One of the common complaints about the carbine concept is that it tries to hard to be a jack of all trades, meaning that it can master none. This was certainly true with the M1 Carbine, which was too weak to kill quickly, but didn't feature fully-automatic fire to overcome that disadvantage. This was corrected in the M2 Carbine, which was almost identical to the M1 save for the featured automatic fire. This problem was avoided in the M4 Carbine by using the same ammunition as its parent weapon. In fact, many soldiers within the most recent war in Iraq attempted to trade in their M-16s for M4's, citing its automatic fire, superior optics, lesser weight, and folding stock.
Carbines are also often carried by soldiers operating within enclosed environments, such as a vehicle, or those working in situations with strict weight limitations, such as paratroopers. | 
