noun

1. the time when the sun in its apparent annual movement along the ecliptic crosses the celestial equator, making night and day of equal length in all parts of the earth: in the Northern Hemisphere the vernal equinox occurs about March 21 and marks the beginning of spring, and the autumnal equinox occurs about September 22 and marks the beginning of autumn
2. either of the two points on the celestial sphere where the sun's path crosses the celestial equator
   also called equinoctial point

Etymology: ME < OFr equinoxe < ML aequinoxium < L aequinoctium < aequus (see equal) + nox, night