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Autumn (sometimes known as fall in North American English) is one of the four temperate seasons. Autumn marks the transition from summer into winter. In the northern hemisphere, the onset of autumn is generally considered to be around September and in the southern hemisphere, its beginning is considered to be around March.
Around this time, deciduous trees shed their leaves. The leaves of the trees change their color to a reddish hue prior to falling. Such colored leaves have come to be colloquially called "fall foliage". 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 during which days get shorter and cooler, the nights get longer, and precipitation gradually increases (in some parts of the world).
The word 'autumn' is derived from the french word 'automne', and became popular in usage for the season since the 16th century. The North American name for the season, 'fall', probably derived as a contraction of the phrase "fall of the leaves", and since became used interchangeably.

Definitions
The word autumn comes from the Old French word autompne (automne in modern French), and was later normalized to the original Latin word autumnus. There are rare examples of its use as early as the 14th century, but it became common only in the 16th, around the same time as fall, and the two words appear to have been used interchangeably.
Before the 16th century harvest was the term usually used to refer to the season. However as more people gradually moved from working the land to living in towns (especially those who could read and write, the only people whose use of language we now know), the word harvest lost its reference to the time of year and came to refer only to the actual activity of reaping, and fall and autumn began to replace it as a reference to the season.
The alternative word fall is now mostly a North American English word for the season. It traces its origins to old Germanic languages. The exact derivation is unclear, the Old English fiæll or feallan and the Old Norse fall all being possible candidates. However, these words all have the meaning "to fall from a height" and are clearly derived either from a common root or from each other. The term only came to denote the season in the 16th century, a contraction of Middle English expressions like "fall of the leaf" and "fall of the year".
During the 17th century, English immigration to the colonies in North America was at its peak and the new settlers took their language with them. While the term fall gradually obsolesced in Britain, it became the more common term in North America, where autumn is nonetheless preferred in scientific and, often, literary contexts.

Etymology
Many ancient civilizations (such as the Amerindians and the ancient Hebrews) computed the years by autumns, It is also symbolised by the Metal element in Chinese astrology.

Historic usage

In popular culture
Autumn's association with the transition from warm to cold weather, and its related status as the season of the primary harvest, has dominated its themes and popular images. In Western cultures, personifications of autumn are usually pretty, well-fed females adorned with fruits, vegetables and grains and wheat that ripen at this time. Most ancient cultures featured autumnal celebrations of the harvest, often the most important on their calendars. Still extant echoes of these celebrations are found in the late-autumn Thanksgiving holiday of the United States, the Jewish Sukkot holiday with its roots as a full moon harvest festival of "tabernacles" (huts wherein the harvest was processed and which later gained religious significance), the many North American Indian festivals tied to harvest of autumnally ripe foods gathered in the wild, the Chinese Mid-Autumn or Moon festival, and many others. The predominant mood of these autumnal celebrations is a gladness for the fruits of the earth mixed with a certain melancholy linked to the imminence of harsh weather.
This view is presented in Keats' poem To Autumn where he describes the season as a time of delightful growth, a seemingly endless time of 'fruitfulness'.

Melancholy association
Especially in the US, autumn is also associated with the Halloween season (which in turn was influenced by Samhain, a Celtic autumn festival), and with it a widespread marketing campaign that promotes it. The television, film, book, costume, home decoration, and confectionery industries use this time of year to promote products closely associated with such holiday, with promotions going from early September to 31 October, since their themes rapidly lose strength once the holiday ends, and advertising starts concentrating on Christmas.
For the American film industry, the autumn season, which begins on the weekend following Labor Day and ends in early November, is the shortest and least profitable season of the movies. It follows the season of summer "blockbusters" and precedes the crowded end-of-year schedule of movies intended for award consideration.

Tourism

Axial tilt
Color change in leaves
Effect of sun angle on climate
Equinox
Indian summer
Temperate