What is "personal defense weapon"

alt. a compact fully-automatic or semi-automatic firearm similar to a submachine gun but firing a rifle round n. a compact fully-automatic or semi-automatic firearm similar to a submachine gun but firing a rifle round

A personal defense weapon (PDW) is a class of compact magazine-fed, self-loading, select-fire firearm – essentially a hybrid between a submachine gun and a carbine, retaining the compact size and ammunition capacity of the former while adding the stopping power, accuracy and penetration of the latter. Most PDWs fire a small-caliber, high-velocity bottleneck cartridge, resembling a small or short rifle round. This gives the PDW better range, accuracy and armor-penetrating capability than submachine guns, which fire pistol-caliber cartridges.

The name describes the type's original role: as a compact but powerful defensive weapon that can be carried by troops behind the front line such as military engineers, drivers, artillery crews or signallers. These soldiers may be at risk of encountering the enemy, but rarely enough that a long-barrel weapon would be an unnecessary burden during their normal duties. Because of their light weight and controllability, they have also been used by special forces and by heavily-armed police.