Definition - the season between summer and winter; also, a period of maturity or incipient decline
Autumn is more confusing, linguistically speaking, than most of the other seasons. It has had multiple titles (fall is common in the U.S., and it's also been known as harvest and harvest-time), and the exact dates of the season vary some. Autumn can be defined as the time extending from the September equinox to the December solstice, or as "the season in the northern hemisphere comprising the months of September, October, and November" (in the southern hemisphere it runs from the March equinox to the June solstice). And to confuse things a bit more in British use it commonly refers to the months of August through October.
Summer has followed after Spring;
Now Autumn is so shrunk and sere,
I scarcely think a sadder thing
Can be the Winter of my year.
— Christina Georgina Rossetti, The Complete Poems of Christina Rossetti, 1998