Point at which on the celestial sphere that the equator and the Ecliptic intersect. Generally the autumnal equinox occurs on or about September 23 in the northern hemisphere; this also signifies spring in the southern hemisphere.
[Encyclopedia of Astronomy; Satterthwaite, Gilbert; 29; 1971; St. Martins Press; NY.] [ Glossary of Astronomy and Astrophysics; Hopkins, Jeanne; 51; 1976; University of Chicago; Chicago.]
Source: Atmospheric Chemistry Glossary