The Collaborative International Dictionary of English v.0.48:

Autumn \Au"tumn\, n. [L. auctumnus, autumnus, perh. fr. a root
   av to satisfy one's self: cf. F. automne. See Avarice.]
   1. The third season of the year, or the season between summer
      and winter, often called "the fall." Astronomically, it
      begins in the northern temperate zone at the autumnal
      equinox, about September 23, and ends at the winter
      solstice, about December 23; but in popular language,
      autumn, in America, comprises September, October, and
      November.
      [1913 Webster]

   Note: In England, according to Johnson, autumn popularly
         comprises August, September, and October. In the
         southern hemisphere, the autumn corresponds to our
         spring.
         [1913 Webster]

   2. The harvest or fruits of autumn. --Milton.
      [1913 Webster]

   3. The time of maturity or decline; latter portion; third
      stage.
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            Dr. Preston was now entering into the autumn of the
            duke's favor.                         --Fuller.
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            Life's autumn past, I stand on winter's verge.
                                                  --Wordsworth.
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WordNet (r) 3.0 (2006):

autumn
    n 1: the season when the leaves fall from the trees; "in the
         fall of 1973" [syn: fall, autumn]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "autumn":
   aestival, arctic, autumnal, boreal, brumal, canicular, equinoctial,
   fall, harvest, harvest home, harvest time, hibernal, hiemal,
   midsummer, midwinter, out of season, seasonal, solstitial, spring,
   springlike, summer, summerlike, summerly, summery, vernal, winter,
   winterlike, wintery, wintry