Meaning

noun either of two times of the year when the sun crosses the plane of the earth's equator and day and night are of equal length

noun (astronomy) either of the two celestial points at which the celestial equator intersects the ecliptic

originPIE word *nókʷts From Middle English equinox, equinoxe, equynox (“one of the two periods in the year when the day and night are of equal length, equinox; either the zodiac sign Aries or Libra, in which the sun crosses the celestial equator”), from Old French equinoce, equinoxe (modern French équinoxe), or from its etymon Medieval Latin ēquinoxium, ēquinoctium, from Latin aequinoctium (“equinox”),…

Etymology from Wiktionary, CC BY-SA 3.0