Merriam-Webster Collegiate Dictionary - equinox

 noun  Etymology: Middle English, from Anglo-French or Medieval Latin; Anglo-French equinocce, from Medieval Latin ~ium, alteration of Latin aequinoctium, from aequi- equi- + noct-, nox night — more at night  Date: 14th century  1. either of the two points on the celestial sphere where the celestial equator intersects the ecliptic  2. either of the two times each year (as about March 21 and September 23) when the sun crosses the equator and day and night are everywhere on earth of approximately equal length