Equinox (noun)

1. Either of the two moments in the year when the sun is exactly above the equator and day and night are of equal length.
2. The time or date when this occurs.
3. An event or condition of balance or equality.

Origin:

Middle english equinox, from old french equinox, from latin aequinoctium, from aequi- equal + nox night.

Examples:

1. The vernal equinox is the moment in the year when the sun is exactly above the equator and day and night are of equal length.
2. The autumnal equinox is the other moment in the year when the sun is exactly above the equator and day and night are of equal length.
3. The equinox of March 20th or 21st is known as the Vernal Equinox, marks the first day of spring in the Northern Hemisphere.
4. The equinox of September 22nd or 23rd is known as the Autumnal Equinox, marks the first day of fall in the Northern Hemisphere.
5. The equinox is an important time for many cultures, and many ancient cultures built structures such as stone circles to track the movements of the sun and the stars during the equinoxes.