From the Wiki

We are entering my second favorite season of the year.. So I looked it up in Wikipedia to learn about the word.

The word autumn comes from the Old French word autompne (automnein modern French), and was later normalised to the original Latin wordautumnus. There are rare examples of its use as early as the 12th century, but it became common by the 16th century.

Before the 16th century, harvest was the term usually used to refer to the season, as it is common in other West Germanic languages to this day (cf. Dutch herfst and German Herbst). 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 autumn, as well as fall, began to replace it as a reference to the season.

The alternative word fall for the season traces its origins to old Germanic languages. The exact derivation is unclear, the Old English fiæll orfeallan 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 came to denote the season in 16th century England, a contraction of Middle English expressions like "fall of the leaf" and "fall of the year".

It is determined by the Fall Equinox which is on Friday this year.  To read about it Click here.