The Collaborative International Dictionary of English v.0.48:

Autumnal \Au*tum"nal\, a. [L. auctumnalis, autumnalis: cf. F.
   automnal.]
   1. Of, belonging to, or peculiar to, autumn; as, an autumnal
      tint; produced or gathered in autumn; as, autumnal fruits;
      flowering in autumn; as, an autumnal plant.
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            Thick as autumnal leaves that strow the brooks
            In Vallombrosa.                       --Milton.
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   2. Past the middle of life; in the third stage.
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            An autumnal matron.                   --Hawthorne.
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   Autumnal equinox, the time when the sun crosses the
      equator, as it proceeds southward, or when it passes the
      autumnal point.

   Autumnal point, the point of the equator intersected by the
      ecliptic, as the sun proceeds southward; the first point
      of Libra.

   Autumnal signs, the signs Libra, Scorpio, and Sagittarius,
      through which the sun passes between the autumnal equinox
      and winter solstice.
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WordNet (r) 3.0 (2006):

autumnal
    adj 1: of or characteristic of or occurring in autumn; "the
           autumnal equinox"; "autumnal fruits" [ant: summery,
           vernal, wintery, wintry]
    2: characteristic of late maturity verging on decline; "a serene
       autumnal mood"