The term "Indian summer" has been commonplace in America for more than two centuries and is used in other countries as well. The team has an uncertain origin, but experts generally agree that it is not a pejorative term.

The term Indian summer is used to describe an unseasonably warm, dry and calm weather pattern or period that follows a period of colder weather. Commonly in the U.S. and in other Northern Hemisphere countries, an Indian summer occurs as autumn progresses, at times even after the first frost of that season. During this short return to warmer weather, hazy or smoky generally wind-free days give way to clear and chilly nights. Often, leaves have started to turn colors and/or fall from trees.

The central and eastern states of the U.S. more commonly have an Indian summer because their weather patterns include more radical shifts in temperature from summer to autumn.

The first written reference to the term is believed to be in the book Letters from an American Farmer, by J.H. St. John de Crèvecoeur in 1778:

"Then a severe frost succeeds which prepares it to receive the voluminous coat of snow which is soon to follow; though it is often preceded by a short interval of smoke and mildness, called the Indian Summer."

The term would have been in relatively common use in conversation before then, in both the U.S. and Canada, and was certainly in more common use in both verbal and written contexts in the 19th Century. Noted American authors John Greenleaf Whittier and Oliver Wendell Holmes included the phrase in their works.

The term is generally accepted as meaning a short-term return of summer, so the use of the word "summer" is accepted readily. But why "Indian"? Scholars do not agree, although they do nearly universally say that it is not a reference to the country of India. The most commonly found explanation for the "Indian" part of an Indian summer is a reference to the "Indians," or, more accurately, Native Americans.

Having lived much longer than Europeans in North America, the peoples of the Native American tribes would have been familiar with the concept of a brief resurgence of summer-like weather in the middle of an otherwise autumnal weather period. They would not have used the term themselves, but they would have explained the concept to Europeans who were arriving in droves after the explorations of Christopher Columbus, Henry Hudson, Marquette & Joliet, and others–not to mention to the settlements at Jamestown, Plymouth, and increasing numbers of others along the eastern seaboard.

Other possible explanations for the term:

• It was the time of year that Native Americans in certain parts of North America harvested their crops.
• It was the time of year in which some Native Americans found the weather conditions ideal for hunting, with an eye toward stocking up for the coming winter. Common practice was to set fire to dry grass, to help obscure the hunters' approach to animal prey.
• It was the time of year in which some European settlements incurred a resurgence of visits and/or attacks from nearby Native Americans.

The term Indian summer is used in countries other than the U.S. The U.K. for a long time used the term Saint Martin's Summer, in conjunction with St. Martin's Day, celebrated in that country on November 11. Other Western European countries used that term for a time as well. Another similar term was St. Luke's Summer, after St. Luke's Day, which is October 18. Several Western European countries now instead use the term Indian summer.

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