A weapon that is capable of burst-fire and/or full-auto-fire.
Semi-automatics are not able to fire bursts or continuously, so they can’t be assault weapons: assault requires the firepower, it’s why the capacity exists. A semi-auto is not a capable battle rifle because it is single-shot. It may be self-cocking and self-reloading, but it is not capable of the rapid rate of fire that field combat use demands.
Yes, that means there are NO semiautomatic assault rifles, no matter *what* they look like.