Definition of autumnal equinox

1) 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.

source: [Encyclopedia of Astronomy; Satterthwaite, Gilbert; 29; 1971; St. Martins Press; NY.] [ Glossary of Astronomy and Astrophysics; Hopkins, Jeanne; 51; 1976; University of Chicago; Chicago.]