equinox

n. Time or date (twice each year) at which the sun crosses the celestial equator, when day and night are of equal length. [latin nox noctis night]

n. The time when the sun enters one of the equinoctial points, that is, about March 21 and September 22. See Autumnal equinox, Vernal equinox, under Autumnal and Vernal.