The term "Indian summer" refers to a period of unseasonably warm, dry weather that occurs in autumn, typically after the first frost. The precise origin of the term is uncertain, but it is generally believed to have originated in the eastern United States. One theory suggests early settlers associated the warm, hazy weather with Native American activities such as hunting or gathering additional food for winter, as Native Americans may have used this weather to extend their outdoor activities. Another explanation is that the term might have arisen from the sight of hazy sun rays resembling Native American campfires. The phrase was first recorded in a letter from 1778 by John de Crevecoeur, who described a warm spell after rain in the Mohawk region as "Indian summer".

The phrase later spread to the UK in the early 19th century, where it is used similarly, though Europe had earlier related concepts such as "Saint Martin's summer" referring to warm weather around mid-November. The term sometimes also carries metaphorical meanings referring to a late revival of warmth or happiness.

In sum, "Indian summer" is called so because it was noted in regions inhabited by Native Americans and was possibly linked to their seasonal activities during these warm autumn spells or their use of fire, but the exact historical reason remains a bit unclear and debated.