equinox

Equinox comes from Latin aequinoctium, formed from aequus ("equal") and nox ("night"), joined with the abstract-noun suffix -ium. It reached English through Old French equinoce/equinoxe or Medieval Latin ēquinoxium, entering Middle English as equinox or equynox.

The Middle English word already carried both senses it has today: the moment of equal day and night, and the zodiac sign (Aries or Libra) where the sun crosses the celestial equator. Its arrival pushed out the native Old English term efnniht, literally "evennight."

Nox, the Latin root for "night," traces back to the Proto-Indo-European *nókʷts, the same ancient root behind the idea of night itself.