define the meaning of the words "assault weapon"

Every time President Obama and the Democratic party have been asked what their plan is about gun rights they always mention that they are going to ban "military style assault weapons". Well, what exactly is a "military style assault weapon"? In the US it is already illegal to purchase a military grade assault rifle. An assault rifle is fully automatic(pull trigger once=multiple rounds fire) and is completely illegal to purchase. Most guns he is referring to are semi-automatic(pull trigger=1 bullet until pulled again). This defines HUNTING RIFLES NOT MILITARY RIFLES. Is it banning cosmetic appearance? That's banning pieces of metal and plastic that do absolutely nothing towards the performance of the gun.So we petition Obama to specify his definition of an "assault weapon"