Autumn (also known as fall in North American English) is one of the four temperate seasons, the transition from summer into winter. In the temperate zones, autumn is the season during which most crops are harvested, and deciduous trees lose their leaves. It is also the season where days rapidly get shorter and cooler, the nights rapidly get longer, and of gradually increasing precipitation in some parts of the world.

Astronomically, some Western countries consider autumn to begin with the autumnal equinox (around September 23) in the Northern hemisphere, and the vernal equinox (March 21) in the southern hemisphere, ending with the winter solstice (around December 21) in the Northern hemisphere and the summer solstice (June 21) in the Southern hemisphere. Such conventions are by no means universal, however. An exception to these definitions is found in the Irish Calendar which still follows the Celtic cycle, where Autumn is counted as the whole months of August, September and October. In Chinese astronomy, the autumnal equinox marks the middle of autumn, which is deemed to have begun around the time of Liqiu (around August 7).

On the other hand, meteorologists count the entire months of March, April and May in the Southern hemisphere, and September, October and November in the Northern hemisphere as autumn.

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