Depends on who you ask I guess. There are actual definitions out there. There is a Wikipedia page. There is a U.S. Army definition of an assault rifle. The term is definitely out there. An AR-15 (Armalite -15) doesn’t tend to fit these definitions as they don’t have a full auto mode. According to one source, they can fire 45 rounds per minute. 400/minute with a bump stock. Assault Rifles are often confused with “assault weapons” which is a legal definition that can change based on state but generally fits AR-15 style rifles - semi automatic, large detachable magazine, pistol style grip.

https://en.wikipedia.org/wiki/Assault_rifle

https://en.wikipedia.org/wiki/Assault_weapon

I knew none of this before looking it up now. I’m not the one to debate with, just relaying info from some Google searching.