equinox (n.)
c. 1400, "point at which the sun crosses the earth's equator, making day and night of equal length everywhere," from Old French equinoce (12c.) or directly from Medieval Latin equinoxium "equality of night (and day)," from Latin aequinoctium, usually in plural, dies aequinoctii "the equinoxes," from aequus "equal" (see equal (adj.)) + nox (genitive noctis) "night" (see night). The Old English translation was efnniht. Related: Equinoctial.