e·qui·nox

  [ee-kwuh-noks, ek-wuh-]

noun
1.
the time when the sun crosses the plane of the earth's equator, making night and day of approximately equal length all over the earth and occurring about March 21 
(vernal equinox or spring equinox)  and September 22 (autumnal equinox)

The equinox is one of the two times in a year when the sun crosses the plane of the earth's equator, and day and night are of equal length. These days are often celebrated and even worshipped in some traditions, although an equinox is actually a point in time — not an entire day.

(from Latin, meaning "equal night"





 

source: http://www.vocabulary.com/dictionary/equinox