The Collaborative International Dictionary of English v.0.48:

Equinox \E"qui*nox\, n. [OE. equinoxium, equenoxium, L.
   aequinoctium; aequus equal + nox, noctis, night: cf. F.
   ['e]quinoxe. See Equal, and Night.]
   1. The time when the sun enters one of the equinoctial
      points, that is, about March 21 and September 22. See
      Autumnal equinox, Vernal equinox, under Autumnal and
      Vernal.
      [1913 Webster]

            When descends on the Atlantic
            The gigantic
            Stormwind of the equinox.             --Longfellow.
      [1913 Webster]

   2. Equinoctial wind or storm. [R.] --Dryden.
      [1913 Webster]

WordNet (r) 3.0 (2006):

equinox
    n 1: either of two times of the year when the sun crosses the
         plane of the earth's equator and day and night are of equal
         length
    2: (astronomy) either of the two celestial points at which the
       celestial equator intersects the ecliptic [syn: equinoctial
       point, equinox]