The word autumn came into general use around the 16th century, replacing the name harvest for the whole season. Autumn is derived from the French, which came from the Latin autumnus, the Roman name for this season.
Fall is a Germanic word that also came into use around the 16th century. It is thought to refer to the season's falling leaves and fruit, and to nature's decline as winter approaches.
Beginning in the 17th century, English-speaking emigrants took both words with them to the New World. In North America, fall became the more common word, while autumn gained the upper hand in Britain, as well as in Australia and New Zealand. In general, then, Americans usually say fall, while the British say autumn—and Canadians say both.
What's the difference?
How do we choose which word to use? Here are some guidelines. For most Canadians, fall is the informal, everyday choice. Use fall when speaking—autumn seems overly formal and a bit pretentious in most everyday contexts.
In writing, both autumn and fall are correct. But when we need a more formal word (or one with two syllables), we speak and write about autumn.