1. The two times of the year when the sun crosses the equator and night and day are of equal length; and it usually occurs on March 21st (spring equinox) and September 23 (fall equinox).
2. One of two points at which the ecliptic cuts the celestial equator.

The vernal equinox is the point in which the sun, traveling on the ecliptic, crosses the celestial equator from south to north.

The intersection of the ecliptic and the celestial equator that the sun passes each year on its way to southern (negative) declinations which occurs on September 23.

The equinox, or celestial equator, which occurs about March 21.