vernal equinox

1. The point at which the ecliptic intersects the celestial equator, the sun having a northerly motion.
2. The moment at which the sun passes through the vernal equinox, about March 21, marking the beginning of spring in the Northern Hemisphere.

astronomy The celestial point on the extended line passing from Earth through the Sun at the moment of the northern spring equinox, and the accepted starting moment of the tropical year and of the zodiac (q.v. for further explanation), hence the zero point for expressing right ascension and celestial longitude (see latitude). It is a slowly moving point relative to the stars, originally in the star pattern Aries, which in classical times signified the opening of the year that began at that equinox.

The noun has 2 meanings:

Meaning #1: March 21
  Synonyms: March equinox, spring equinox
  Antonym: autumnal equinox (meaning #1)

Meaning #2: (astronomy) the equinoctial point that lies in the constellation of Pisces