Indian summer, Saint Martin's summer - a period of unusually warm weather in the autumn

Sep, Sept, September - the month following August and preceding October

Oct, October - the month following September and preceding November

Nov, November - the month following October and preceding December

autumnal equinox, fall equinox, September equinox - September 22

time of year, season - one of the natural periods into which the year is divided by the equinoxes and solstices or atmospheric conditions; "the regular sequence of the seasons"