assault weapon

Short definition from Wiktionary

Noun

1. 1
   Any select-fire firearm that allows semi-automatic and fully automatic operation, and is used or was once used by a military organization.
2. 2
   A semi-automatic firearm that resembles a military weapon.
3. 3
   A semi-automatic firearm that resembles a military weapon: A semi-automatic weapon that has a folding stock, a muzzle flash suppressor, a bayonet mount, or a pistol grip.
4. 4
   A rocket launcher designed to attack fortified structures, such as a Mk 153 Shoulder-Launched Multipurpose Assault Weapon (aka SMAW).