An Indian summer is a period of unseasonably warm, dry weather that sometimes occurs in autumn in temperate regions of the Northern Hemisphere. Several sources describe a true Indian summer as not occurring until after the first frost, or more specifically the first "killing frost."

The late 19th-century lexicographer Albert Matthews made an exhaustive search of early American literature in an attempt to discover who coined the expression. The earliest reference he found dated to 1851. He also found the phrase in a letter written in England in 1778, but discounted that as a coincidental use of the phrase.

Later research showed that the earliest known reference to Indian summer in its current sense occurs in an essay written in the United States around 1778 by J. Hector St. John de Crèvecœur, describing the character of autumn and implying the common usage of the expression

Great rains at last replenish the springs, the brooks, the swamp and impregnate the earth. 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. This is in general the invariable rule: winter is not said properly to begin until those few moderate days & the raising of the water has announced it to Man.

The essay was first published in French around 1788, but remained unavailable in the United States until the 1920s.

Although the exact origins of the term are uncertain, it was perhaps so-called because it was first noted in regions inhabited by Native Americans, or because the natives first described it to Europeans, or it had been based on the warm and hazy conditions in autumn when Native Americans hunted. John James Audubon wrote about "The Indian Summer that extraordinary phenomenon of North America" in his journal on November 20, 1820. He mentions the "constant Smoky atmosphere" and how the smoke irritates his eyes. Audubon suspects that the condition of the air was caused by "Indians, firing the Prairies of the West". Audubon also mentions in many other places in his writings the reliance Native Americans had on fire. At no point does Audubon relate an Indian summer to warm temperatures during the cold seasons.

Because the warm weather is not a permanent gift, a connection has been made to the pejorative term Indian giver. Native-American legends mention the god or "Life-Giver" bestowing warm autumnal weather to various warriors or peoples, enabling them to survive after great misfortune, such as loss of crops.

Weather historian William R. Deedler wrote that "Indian summer" can be defined as "any spell of warm, quiet, hazy weather that may occur in October or November", though he noted that he "was surprised to read that Indian summers have been given credit for warm spells as late as December and January". Deedler also noted that some writers use Indian summer in reference to the weather in only New England, "while others have stated it happens over most of the United States, even along the Pacific coast".