The old definition? Before liberal scare tactics? Something approximating this:
A selective-fire rifle* with a detachable magazine, chambered in an intermediate caliber**.
*Automatic fire; i.e. single/auto, single/burst, burst/auto, single/burst/auto, etc. Single/Safe doesn't count.
**A caliber less powerful than a typical battle rifle cartridge, but more powerful than a typical pistol cartridge.
Those are the key terms/qualities, if not the original organization of the sentence.
The original "assault rifle" was the StG44 during WWII: it was chambered in 7.92x33, which was more powerful than the Luger P08 or Walther in 9x19, and less powerful than a Gehwehr in 7.92x57.
One could argue that the M2 Carbine was the first US "assault rifle" - filling a place between pistols or submachine guns in 32acp/45acp, and rifles in 30-06 (later between 45acp and 7.62x51). Some might argue it's more of a PDW, especially given how it was issued - I'm not sure I can take sides on that argument... It was first issued in 1945 and saw service right through the 1960s.
The AK platform (unsurprisingly "finished" in 1947, though issued in 1948) is definitely an "assault rifle", as it was/is issued in 7.62x39 or 5.45x39, and fills a place between 9x18 and 7.62x54R. Of course the M16/M4 platform issued in 5.56x45 is an "assault rifle", filling the niche between 45acp (later 9x19) and 7.62x51 - the AR platform was issued by the Airforce in 1963, and not issued by the Army until 1965; a full 5-7 years after the semi-auto AR15 was created, and 8-10 years after the semi-auto AR10 on which it was based was created.
Of course there are countless others, but those are good talking points that most normies would "get". Everyone knows "M16" and "AK47" right? The big thing is it must go bang more than once when you pull the trigger once, or be capable of doing so by flipping a lever, pushing a button, etc. - otherwise it's not an "assault rifle". That's the primary difference from an MSR.