Dry, warm
weather in
October or
November. Traditionally the phrase is applied to warm weather after the first
frost of
autumn.
In San Francisco, which doesn't get frost until winter (if at all), the phrase can refer to the annual heat wave at the end of September/beginning of October, where grateful residents enjoy the 90 degree plus temperatures which follow the foggy, cold summer of the city's Mediterranean climate.
First usage is cited in America in a 1778 letter by Frenchman St. John de Crevecoeur:
"Sometimes the rain is followed by an interval of calm and
warmth which is called the Indian Summer; its characteristics
are a tranquil atmosphere and general smokiness. Up to this
epoch the approaches of winter are doubtful; it arrives about
the middle of November, although snows and brief freezes often
occur long before that date."
Why "
Indian" summer? Lots of contrasting theories, many involving
Native American seasonal
hunting, as well as the loading of ships to cross the
Indian ocean during this fair weather time of year.
Sources: http://www.crh.noaa.gov/dtx/i-summer.htm;
also going outside, licking my finger, and holding it up and squinting skyward