Indian summer is a period of unusually warm and dry weather. It happens in autumn in the Northern Hemisphere. The United States National Weather Service describes Indian summer as weather conditions which are sunny and clear with higher than usual temperatures. That occurs between late September and the middle of November.
It is not known where the term came from. It may have come from regions inhabited by Native Americans (who used to be called Indians), or because the Native Americans first described it to Europeans. It also may have been based on the warm and hazy conditions in autumn when Native Americans hunted.
Media related to Indian summer at Wikimedia Commons