What season do Americans call 'fall'?

Autumn and fall are used interchangeably as words for the season between summer and winter. Both are used in American and British English, but fall occurs more often in American English. Autumn is considered the more formal name for the season.

The older of the two words is autumn, which first came into English in the 1300s from the Latin word autumnus. (Etymologists aren't sure where the Latin word came from.) It had extensive use right from its first appearance in English writing, and with good reason: the common name for this intermediary season prior to the arrival of autumn was harvest, which was potentially confusing, since harvest can refer to both the time when harvesting crops usually happens (autumn) as well as the actual harvesting of crops (harvest). The word autumn was, then, a big hit.

Names for the season didn't just end with autumn, however. Poets continued to be wowed by the changes autumn brought, and in time, the phrase "the fall of the leaves" came to be associated with the season. This was shortened in the 1600s to fall.