Assault rifle refers to the calibre and application of a rifle.
The smaller 5.56 round is an assault rifle round, this is to distinguish it from the previous larger battle rifle rounds.
The AR-15 was designed with select fire. The ones sold to civilians don't have this capability because it's illegal.
The only people that define these in such a way as you have a gun nuts. Trying to hide the fact that people are selling and marketing a weapon of war to civilians in a peaceful country.