Assault weapon is a non-technical term referring to any of a broad category of firearms with certain features, including some semiautomatic rifles, some pistols, and some shotguns. There are a variety of different statutory definitions of assault weapons in local, state, and federal laws in the United States that define them by a set of characteristics they possess. Using lists of physical features or specific firearms in defining assault weapons in the U.S. was first codified by the language of the now-expired 1994 Federal Assault Weapons Ban.

Very generally speaking, a semi-automatic firearm is defined by these laws as an assault weapon if it has both a detachable magazine and a pistol grip, sometimes in conjunction with other features such as a folding stock or a flash suppressor. Assault weapons are often similar in appearance to military firearms, but are capable of firing only once each time the trigger is pulled.

Whether or not assault weapons should be legally restricted more than other firearms, how they should be defined, and even whether or not the term assault weapon should be used at all, are questions subject to considerable debate as part of the arguments of gun politics in the United States.