1. (Also called fall, especially in the United States. ) The season of the year that is the transition period from summer to winter, occurring as the sun approaches the winter solstice. In popular usage and for most meteorological purposes, autumn is customarily taken to include the months of September, October, and November in the Northern Hemisphere, and March, April, and May in the Southern Hemisphere. 2. Astronomically, the period extending from the autumnal equinox to the winter solstice.