No one really knows for sure who created the definition of "assault weapon" but one thing for sure is that Josh Sugarman of everyone's favorite anti-gun organization, VPC (also the orginators of "cop killer bullets", "x-ray invisible plastic guns", and now "sniper rifles") did it's best to propagandize the term.
Here are some of his quotes pertaining to "assault weapons":
"Assault weapons... are a new topic. The weapons' menacing looks, coupled with the public's confusion over fully-automatic machine guns versus semi-automatic assault weapons -- anything that looks like a machine gun is assumed to be a machine gun -- can only increase the chance of public support for restrictions on these weapons. In addition, few people can envision a practical use for these weapons."
"If police continue to call for assault weapons restrictions, and the NRA continues to fight such measures, the result can only be a further tarnishing of NRA's image in the eyes of the public, the police, and NRA members. The organization will no longer be viewed as the defender of the sportsman, but as the defender of the drug dealer."
"Efforts to restrict assault weapons are more likely to succeed than those to restrict handguns."
"... many Americans do believe that handguns are effective weapons for home-defense and the majority of Americans mistakenly believe the Second Amendment of the Constitution guarantees the individual right to keep and bear arms. Yet, many who support the individual's right to own a handgun have second thoughts when the issue comes down to assault weapons. Assault weapons are often viewed the same way as machine guns and `plastic' firearms -- a weapon that poses such a grave risk that it's worth compromising a perceived constitutional right."