Wiktionary
n. 1 (context US English) Any select-fire firearm that allows semi-automatic and fully automatic operation, and is used or was once used by a military organization. 2 (context US English) A semi-automatic firearm that resembles a military weapon. 3 # (context New York State English) A semi-automatic weapon that has a folding stock, a muzzle flash suppressor, a bayonet mount, or a pistol grip.
Wikipedia
Assault weapon is a term used in the United States to define some types of firearms. The definition varies among regulating jurisdictions, but usually includes semi-automatic firearms with a detachable magazine and a pistol grip, and sometimes other features such as a flash suppressor or barrel shroud. Some firearms are specified by name. At the time that the now-defunct Federal Assault Weapons Ban passed in 1994, the U.S. Justice Department said, "In general, assault weapons are semiautomatic firearms with a large magazine of ammunition that were designed and configured for rapid fire and combat use." The origin of the term has been attributed to legislators, gun control groups, the media, and the firearms industry itself. It is sometimes conflated with the term " assault rifle", which refers to selective-fire military rifles that can fire in automatic and / or burst mode.
After the December 2012 Sandy Hook Elementary School shooting, many news organizations ran stories about assault weapons, explaining their varying definitions and presenting varying opinions about whether or not they should be banned again on the federal level.
Assault weapon is a legal, political and commercial term used to define specific firearms marketed to civilians, paramilitary, and police agencies.