n. 1 the time or date (twice each year) at which the sun crosses the celestial equator, when day and night are of equal length. 2 = equinoctial point. autumn (or autumnal) equinox about 22 Sept. spring (or vernal) equinox about 20 March.
[ ME f. OF equinoxe or med.L equinoxium for L aequinoctium (as EQUI-, nox noctis night) ]