assault weapon noun4 •(US) A semi-automatic firearm that resembles a military weapon. •(US) A semi-automatic weapon that has a folding stock, a muzzle flash suppressor, a bayonet mount, or a pistol grip. •(US) Any select-fire firearm that allows semi-automatic and fully automatic operation, and is used or was once used by a military organization. •(US) A rocket launcher designed to attack fortified structures, such as a Mk 153 Shoulder-Launched Multipurpose Assault Weapon (aka SMAW). formsassault weapons