Why do we have two words for the same season? Both "fall" and "autumn" are very old words. Autumn, from the Latin word autumnus ("to enrich or furnish with") dates back to the early 14th century. Autumn has always meant the season of plenty or "time of full maturity." But it had to bide its time to win popular acceptance.
For centuries before, English speakers preferred to call the three months before the winter solstice "harvest." In the Middle Ages, the harvest was so important that the Old English word "haerfest" meant not only the reaping and gathering of crops but also the time of year these activities took place. Harvest and autumn were in many ways synonyms. Autumn eventually supplanted harvest in usage, as autumn suggests a wider sense of the season, including its crisp, clear weather. In Britain today, "autumn" is the term of choice.
Other popular words emerged from this third season as well. For example: "Aftermath," meaning result or consequence, used to be "aftermowth," the second mowing or crop in a growing season.
According to custom, the Anglo-Saxons had a harvest activity called the "threscan" where people stamped on stalks of wheat to separate the grain from the chaff. This threshing action moved from barns to the doorways of houses, where people also stamped their feet to clean off their shoes before entering. That's likely how our doorsills were named thresholds.
But, where did fall come from?
By the 18th century, autumn gained another synonym from an Englishman's clever turn of phrase 200 years before. Scholar Roger Ascham listed the four seasons in 1545 as "spring tyme, somer, fall of the leafe, and winter." By 1664, "fall of the leafe" shortened to fall, was gaining modest support among British speakers. American colonists, however, clung to it, and ever since have preferred this more informal way of describing the season.
SOURCES: 'The Encyclopedia of Word and Phrase Origins,' by Robert Hendrickson; 'Webster's Word Histories'; 'Room's Dictionary of Changes in Meaning,' by Adrian Room; 'Brewer's Dictionary of Phrase and Fable,' edited by Ivor Evans; 'The Barnhart Dictionary of Etymology,' by Robert Barnhart; 'Etymological Dictionary of the English Language,' by Walter Skeat; 'Webster's Third New International Dictionary.'