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Assault weapon
Assault weapon or "assault-style weapon" is a nebulous term used in the United States by public officials, media, and gun-control proponents as a dysphemism to refer to any firearms they consider inappropriate for civilian ownership or look particulary menacing. Congress has frequently changed and discussed changing the definition of the term.
Many (but not all) assault weapons share these characteristics:

Semiautomatic rifle, pistol, or shotgun
Ability to accept a detachable magazine
Rapidity of fire due to large magazine capacity
Medium power ammunition
Suitability for law enforcement or military use
Military-style appearance, including features that are of dubious utility to private citizens, such as a grenade launcher even though the actual grenade is not available to civilians.

The Bureau of Alcohol, Tobacco, Firearms, and Explosives defines an "Assault Weapon" (for a rifle) as any semi-automatic rifle with the capability to accept two or more of the following features:

Folding or telescopic stock
Pistol grip protruding conspicuously beneath the stock
Bayonet mount
Flash suppressor or threaded barrel
Grenade launcher

For a pistol as any semi-automatic pistol which has the ability to accept a removeable magazine, and has two or more of the following features:

Magazine that attaches outside of the pistol grip
Threaded barrel capable of accepting a barrel extender, flash suppressor, forward handgrip, or silencer*
Shroud that is attached to, or partially or completely encircles, the barrel and that permits the shooter to hold the firearm with the non-trigger hand without being burned
Manufactured weight of 50 ounces or more when the pistol is unloaded
Semiautomatic version of an automatic firearm

This ban leaves many gray areas, and has no exemption for collectors.
The frequent and erroneous use of the term assault rifle by media and gun control supporters when reporting on or discussing assault weapons has produced a popular and intentional misconception that assault weapons are fully-automatic machine guns. The terms are used interchangeably in an effort to confuse the less-informed public and to get them to oppose "assault-style" firearms.

Assault weapons and their owners are frequently criticized by certain disarmament groups such as the Brady Campaign to Prevent Gun Violence, Million Mom March, and other organizations. Those who would own assault weapons often cite novelty, collectibility, riot control, and civil defense as justification. Those who would outlaw assault weapons cite cultural utilitarianism and other societal justifications such as public safety from a perceived threat to stability presented by private ownership of assault weapons.

Many countries prohibit or heavily restrict the ownership of true assault rifles by private citizens.

See also: assault weapons ban, assault rifle