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 equinox
    n 1: September 22 [syn: autumnal equinox, September equinox,
         fall equinox] [ant: March equinox, spring equinox,
         vernal equinox]
    2: (astronomy) the equinoctial point that lies in the
       constellation of Virgo