|a. Also called (esp US): fall the season of the year between summer and winter, astronomically from the September equinox to the December solstice in the N hemisphere and from the March equinox to the June solstice in the S hemisphere|
|b. (as modifier): autumn leaves|
|2.||a period of late maturity, esp one followed by a decline|
|[C14: from Latin autumnus, perhaps of Etruscan origin]|
|a chattering or flighty, light-headed person.|
|an extraordinary or unusual thing, person, or event; an exceptional example or instance.|
season of the year between summer and winter during which temperatures gradually decrease. It is often called fall in the United States because leaves fall from the trees at that time. Autumn is usually defined in the Northern Hemisphere as the period between the autumnal equinox (day and night equal in length), September 22 or 23, and the winter solstice (year's shortest day), December 21 or 22; and in the Southern Hemisphere as the period between March 20 or 21 and June 21 or 22. The autumn temperature transition between summer heat and winter cold occurs only in middle and high latitudes; in equatorial regions, temperatures generally vary little during the year. In the polar regions autumn is very short. For physical causes of the seasons, see season.
Learn more about autumn with a free trial on Britannica.com.