A submachine gun (SMG) is an air-cooled, magazine-fed, automatic carbine designed to fire pistol cartridges. The term "submachine gun" was coined by John T. Thompson, the inventor of the Thompson submachine gun.
The submachine gun was developed during World War I (1914–1918). At its zenith (1939–1945), millions of SMGs were made. After the war, new SMG designs appeared almost every week. However, by the 1980s, SMG usage decreased. Today, submachine guns have been largely replaced by assault rifles, which have a greater effective range and are capable of penetrating the helmets and body armor used by modern infantrymen. However, submachine guns are still used by military special forces and police SWAT teams for close quarters battle (CQB) because they're "a pistol caliber weapon that's easy to control, and less likely to over-penetrate the target."