Today it’s a matter of regional usage (e.g. North America vs Great Britain) but how did we get here?
Pre-16th century, the season was known as “hærf-est”, basically “harvest”. Interesting to note that this root is still in play in modern Germanic languages (Dutch herfst, German Herbst and Scottish hairst).
The word “autumn” is thought to have come from the ancient Etruscan root autu- which relates to year or end of year. Not surprisingly, you will find at least a trace of this root word in the modern word for fall/autumn in most romance languages.
At some point post-16th century, the word autumn was introduced into the English language and a distinction was made between the season and harvest time. It was likely a confluence of linguistic influences jumping the pond from the continent as well as a move away from agricultural living and less association with actual harvest time. During the same period, the word “fall” was also introduced as an alternative to “hærf-est. It seems clear that the root of “fall” is from the Old Norse, English and German words meaning “to fall” as in from a high place.
This period also coincides with the British colonization of America and the beginning of our linguistic diverging. Given that the new Englanders were in leaf-falling central, it seems apt that the word “fall” stuck here. And, speaking of which, enjoy our beautiful fall!