Autumn is one of the four temperate seasons, marking the transition from summer into winter. In the northern hemisphere, autumn begins in September. It begins in March in the southern hemisphere.

Autumn is also known as fall, because it is the time when deciduous trees (those which lose their leaves seasonally) shed their leaves. Days become shorter and cooler, and nights are longer.

Astronomically, some Western countries consider autumn to begin with the September equinox (around September 23) in the northern hemisphere, and the March equinox (March 21) in the southern hemisphere, ending with the December solstice (around December 21) in the northern hemisphere and the June solstice (June 21) in the southern hemisphere. The Irish calendar, however, still follows the Celtic cycle, where autumn is counted as the whole months of August, September and October. In Chinese astronomy, autumn begins around the time of Liqiu (early August), and the autumnal equinox marks the middle of the season.