vernal equinox

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One of two points at which the ecliptic intersects the celestial equator. At the vernal equinox, the sun is moving along the ecliptic in a northeasterly direction.
• noun The moment at which the sun passes through this point on or about March 21, marking the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun the moment when the sun appears to cross the celestial equator, heading northward
• noun the point where the ecliptic intersects the celestial equator at that time

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (astronomy) the equinoctial point that lies in the constellation of Pisces
• noun March 21