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a period of unusually settled warm weather after the end of summer proper
Indian summer[′in·dē·ən ′səm·ər]
A period, in mid-or late autumn, of abnormally warm weather, generally clear skies, sunny but hazy days, and cool nights; in New England, at least one killing frost and preferably a substantial period of normally cool weather must precede this warm spell in order for it to be considered a true Indian summer; it does not occur every year, and in some years there may be two or three Indian summers; the term is most often heard in the northeastern United States, but its usage extends throughout English-speaking countries.
a period of mild, dry weather occurring in U.S. and Canada in late autumn. [Am. Culture: Misc.]