The Collaborative International Dictionary of English v.0.48:

vernal \ver"nal\ (v[~e]r"nal), a. [L. vernalis, fr. vernus
   vernal, ver spring; akin to Gr. 'e`ar, Skr. vasanta, Icel.
   v[=a]r, and E. Easter, east.]
   1. Of or pertaining to the spring; appearing in the spring;
      as, vernal bloom.
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            And purple all the ground with vernal flowers.
                                                  --Milton.
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   2. Fig.: Belonging to youth, the spring of life.
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            When after the long vernal day of life. --Thomson.
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            And seems it hard thy vernal years
            Few vernal joys can show?             --Keble.
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   Vernal equinox (Astron.), the point of time in each year
      when the sun crosses the equator when proceeding
      northward, about March 21, when day and night are of
      approximately equal duration. The beginning of the Spring
      season.

   Vernal grass (Bot.), a low, soft grass (Anthoxanthum
      odoratum), producing in the spring narrow spikelike
      panicles, and noted for the delicious fragrance which it
      gives to new-mown hay; -- also called sweet vernal
      grass. See Illust. in Appendix.

   Vernal signs (Astron.), the signs, Aries, Taurus, and
      Gemini, in which the sun appears between the vernal
      equinox and summer solstice.
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WordNet (r) 3.0 (2006):

vernal equinox
    n 1: March 21 [syn: vernal equinox, March equinox, spring
         equinox] [ant: September equinox, autumnal equinox,
         fall equinox]
    2: (astronomy) the equinoctial point that lies in the
       constellation of Pisces

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "vernal equinox":
   aphelion, apogee, astronomical longitude, autumnal equinox,
   celestial equator, celestial longitude, celestial meridian, circle,
   colures, ecliptic, equator, equinoctial, equinoctial circle,
   equinoctial colure, equinox, galactic longitude,
   geocentric longitude, geodetic longitude, great circle,
   heliocentric longitude, longitude, meridian, orbit, perigee,
   perihelion, period, small circle, solstice, solstitial colure,
   summer solstice, trajectory, winter solstice, zodiac, zone