A machine pistol is a handgun

Handgun

A handgun is a firearm designed to be held and operated by one hand. This characteristic differentiates handguns as a general class of firearms from long guns such as rifles and shotguns ....

-style, often magazine

Magazine (firearm)

A magazine is an ammunition storage and feeding device within or attached to a repeating firearm. Magazines may be integral to the firearm or removable . The magazine functions by moving the cartridges stored in the magazine into a position where they may be loaded into the chamber by the action...

-fed and self-loading firearm

Firearm

A firearm is a weapon that launches one, or many, projectile at high velocity through confined burning of a propellant. This subsonic burning process is technically known as deflagration, as opposed to supersonic combustion known as a detonation. In older firearms, the propellant was typically...

, capable of fully automatic

Automatic firearm

An automatic firearm is a firearm that loads another round mechanically after the first round has been fired.The term can be used to refer to semi-automatic firearms, which fire one shot per single pull of the trigger , or fully automatic firearms, which will continue to load and fire ammunition...

 or burst fire, and normally chambered for pistol

Pistol

When distinguished as a subset of handguns, a pistol is a handgun with a chamber that is integral with the barrel, as opposed to a revolver, wherein the chamber is separate from the barrel as a revolving cylinder. Typically, pistols have an effective range of about 100 feet.-History:The pistol...

 cartridges. The term is a literal translation of Maschinenpistole, the German term for a hand-held automatic weapon. While the dividing line between machine pistols and compact submachine gun

Submachine gun

A submachine gun is an automatic carbine, designed to fire pistol cartridges. It combines the automatic fire of a machine gun with the cartridge of a pistol. The submachine gun was invented during World War I , but the apex of its use was during World War II when millions of the weapon type were...

s is hard to draw, the term "submachine gun" usually refers to larger automatic firearms scaled down from a full-sized machine gun to fire handgun rounds, while the term "machine pistol" usually refers to a weapon built up from a semi-automatic pistol design.

In a law enforcement context, machine pistols are used by tactical police units such as SWAT

SWAT

A SWAT team is an elite tactical unit in various national law enforcement departments. They are trained to perform high-risk operations that fall outside of the abilities of regular officers...

 teams or hostage rescue teams which are operating inside buildings and other cramped spaces, who need a small, concealable weapon with a high rate of fire.