Autumn is the season that occurs between summer and winter. It is also referred to as fall, particularly in the United States. It occurs at different points throughout the year in the Northern and Southern Hemispheres of Earth. In the Northern Hemisphere, autumn occurs from September through December. In the Southern Hemisphere, it occurs from March through June.
As autumn progresses to winter, the daylight hours get shorter, the temperature decreases, and the length of nighttime increases.
Perhaps the most distinguishing feature of autumn is the changing of the leaves. During autumn of each year, the leaves of deciduous plants (like trees) change color and fall. Thus, in regions where there are many deciduous plants, the changing of the seasons is very apparent.
Why are the seasons different around the globe?
The Earth rotates about its axis as well as around the Sun. Because the axis of rotation of Earth is tilted (~23°) relative to the plane formed by its orbit around the Sun, different parts of Earth experience varying amounts of sunlight as the Earth orbits the Sun over the course of a year. This is reflected in the change of the seasons as well as the fact that the seasons occur at different times of the year in the Northern and Southern hemispheres based on Earth's position relative to the sun along its orbit.
During the months of May, June, and July, the Northern Hemisphere faces the Sun, so these regions get more sunlight throughout that period of time. The same is true of the Southern Hemisphere during the months of November, December, and January.
While the Northern and Southern Hemispheres experience the four seasons at different times, the tropical and subtropical regions of Earth don't really experience the four seasons at all. This is because regions around the Equator see little variation in sunlight throughout the year, so the average temperature does not vary much, unlike regions in the Northern and Southern Hemispheres. Rather than having four seasons throughout the year, tropical regions instead have wet and dry seasons where the amount of precipitation significantly varies.
Autumn vs fall
Autumn and fall both refer to the season between summer and winter. Autumn is the term that is used throughout most of the world. In the United States, fall is the preferred term. However, the terms can be used interchangeably.