- equinox late 14c., from O.Fr. equinoce (12c.) or directly from M.L. equinoxium "equality of night (and day)," from L. aequinoctium "the equinoxes," from aequus "equal" (see EQUAL (Cf. equal)) + nox (gen. noctis) "night" (see NIGHT (Cf. night)). The O.E. translation was efnniht. Related: Equinoctial.
Etymology dictionary. 2014.