Definitions for "Autumnal equinox"
The equinox that occurs on or near September 21 each year. See also equinox vernal equinox
A day of equal day and night that marks the beginning of fall and occurs when the sun passes directly over the equator.
The place in the sky where the Sun crosses the celestial equator moving southwards. The autumnal equinox marks the beginning of autumn and falls on, or near, September 21. See celestial sphere for a helpful diagram.
(astronomy) the equinoctial point that lies in the constellation of Virgo