A personal defense weapon (often abbreviated PDW) is a compact semi-automatic or fully-automatic firearm similar in most respects to a submachine gun, but firing an (often proprietary) armor-piercing round, giving a PDW better range, accuracy and armor-penetrating capability than submachine guns, which fire pistol-caliber cartridges.
The P90 was designed to have a length no greater than a man's shoulder width, in order to be easily carried and maneuvered in tight spaces, such as the inside of an armored vehicle. To achieve this, the weapon's design utilizes the unconventional bullpup configuration, in which the action and magazine are located behind the trigger and alongside the shooter's face, so that there is no wasted space in the stock. The P90's dimensions are also minimized by its unique horizontally mounted feeding system, wherein the box magazine sits parallel to the barrel on top of the weapon's frame. Overall, the weapon has an extremely compact profile.