"Assault" is just a word appended to weapons by the anti-gun lobby. It has no particular or specific meaning other than to engender or play to fears. There are automatic (one trigger pull empties magazine), semi-automatic (one pull, one shot), and single-shot weapons--that's basically it. Automatics that exist legally are the subject of licensing, extensive controls and personal possession is extremely rare. And as to the "scary" weapons, there is effectively no functional difference between a pink-stocked kids beginner .22 and an AR-15--it's one pull, one shot.
I'm starting to think that an assault weapon is apparently anything with a black plastic stock or is aggressive looking.