From Wikipedia: The term assault weapon is a political and legal term used to describe a variety of semi-automatic firearms that have certain features generally associated with military assault rifles. The 1994 Federal Assault Weapons Ban, which expired on September 13, 2004, codified the definition of an assault weapon. It defined the rifle type of assault weapon as a semiautomatic firearm with the ability to accept a detachable magazine, and two or more of the following:

Folding or telescoping stock
Primary pistol grip
Forward grip
Threaded barrel (for a Suppressor, commonly called a silencer)
Barrel shroud