Personal Defense Weapon. Originally used to describe HK's MP5K (MP for Maschine Pistole or Machine Pistol and K for "Kurtz" meaning small). The term has since left being used to describe that weapon and has grown to represent a new class of weaponry that is supposed to have the firepower of a rifle (such as a M16 series rifle in 5.56mm)yet the ease of carry of a submachine gun (like the larger MP5 variants).
Two weapons currently fit such a description, FN Herstal's FNP90 (5.7x28mm)and HK's MP7 (4.6x30mm). Both of these weapons are marketed as being perfect for support staff and pilots where a full size rifle would be too large to be practical