autumnal 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 autumnal equinox, the sun is moving along the ecliptic in a southeasterly direction.
• noun The moment at which the sun passes through this point on or about September 23, marking the beginning of autumn in the Northern Hemisphere and spring 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 southward
• 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 September 22
• noun (astronomy) the equinoctial point that lies in the constellation of Virgo