Etymology
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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.

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Definitions of equinox

equinox (n.)
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;
equinox (n.)
(astronomy) either of the two celestial points at which the celestial equator intersects the ecliptic;
Synonyms: equinoctial point
From wordnet.princeton.edu