submachine gun
A gun that combines the automatic fire capability of a machine gun with the cartridge of a pistol, and is usually between the two in weight and size.
During World War II they were a key element to most military arsenals, but the advent of the assault rifle has relegated them mostly to military vehicle crews, police units and covert operations use.
A subset of submachine guns are machine pistols, which are SMGs similar in size to ordinary pistols (and in many cases, simply full-auto variants of pistols). Since concealability is the goal for machine pistols, they usually have shoulder stocks that are folding, collapsible, detachable or even completely absent.