Definition of "autumnal equinox" []

• The point at which the ecliptic intersects the celestial equator, the sun having a southerly motion. (noun)
• The moment at which the sun passes through the autumnal equinox, about September 23, marking the beginning of autumn in the Northern Hemisphere. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "This happens twice a year, and the times when the sun passes the equator are called the vernal or spring equinox and the autumnal equinox respectively."