Source : Webster's Revised Unabridged Dictionary (1913)

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.

            Thick as autumnal leaves that strow the brooks In
            Vallombrosa.                          --Milton.

   2. Past the middle of life; in the third stage.

            An autumnal matron.                   --Hawthorne.

   {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.

Source : WordNet®

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