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
"Astronomers mark the vernal equinox as the precise moment each spring when the sun aligns directly over the Earth's equator, resulting in nearly equal hours of daylight and darkness."
(astronomy) either of the two celestial points at which the celestial equator intersects the ecliptic
"The astronomer adjusted her telescope to track the precise moment when the sun crossed one of the celestial points where the equator meets the ecliptic during the equinox."
The intersection of the apparent path of the sun in the sky (the ecliptic) with the celestial equator.
"Astronomers mark the precise moment of the equinox when the sun crosses the celestial equator along its apparent yearly path across the sky."
The word equinox comes from the Latin term aequinoctium, which combines aequus meaning "equal" and nox meaning "night." It originally referred to the two times each year when day and night are of equal length.