Yesterday morning (at 10:21 am), the third season started. To some, fall had just begun. To others, it was when autumn got rolling.
Both groups are right, of course. Unlike spring, summer, and winter, the season we are in right now has two accepted names. But the term that you choose to use? It can even say a bit about where someone is from, too.
America vs. Britain
The plainest way to explain the two terms is that "fall" is an American term, while "autumn" is British. If you're from English Canada — a country that is neighbours with the U.S., but a part of the British Commonwealth — both terms are used regularly. And if you're from Quebec or another French-speaking part of the country? The French name for the season is "automne", which is much closer to the British version.
In the past, there have been other names in play, too. The most significant one was "haerfest" or "harvest" — a reference to the time of year when crops were harvested at the end of the growing season. Spring has also had several different names over the centuries, including "lent" (a reference to a religious holiday during the season), and "ver" (meaning "green" in Latin).
Spring and fall = BFFs
So which term do you use for the third season? A big reason why fall won out in certain parts is because it makes so much sense as a pair to spring. You spring up. You fall down. You spring forward. You fall back. Plants spring to life. Leaves fall to the ground. The two words are perfect together. They actually describe what happens in each season.
Meanwhile, autumn comes from a Latin word autumnus. It comes from a root word that means the "passing of the year." That's a pretty good term, too, but not one that immediately makes as much sense. In a way, Americans choosing fall over autumn is pretty understandable. Most of the differences in American English (such as taking the "u" out of words like "armour") are about removing unnecessary extra bits from words or trying to make things clearer. And as a word, fall's meaning is clear.
Either way, whichever term you prefer, people will know what you mean when you say it. We like to think of it as a special bonus for an already pretty special season.