It was the first example of the Assault Rifle weapon type with the defining characteristics being an intermediate cartridge (between pistol round and rifle round) that allows controllable automatic fire at engagement ranges further than submachine guns.
Most AR15s cannot be considered 'Assault Rifles' as although they are chambered in an intermediate cartridge, they are for the most part semi-automatic and not capable of automatic fire.
Note that 'Assault Rifle' does not seem to be a ATF defined term, they always use the term Machine Gun for automatic weapons. The term "Assault Rifle" is more commonly associate d with anti-gun organizations as it has become a emotionally charged word