Real Dictionary

equinox
countable noun: An equinox is one of the two days in the year when day and night are of equal length.

equinox in American English
the time when the sun in its apparent annual movement along the ecliptic crosses the celestial equator, making night and day of equal length in all parts of the earth: in the Northern Hemisphere the vernal equinox occurs about March 21 and marks the begin
noun: 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)

equinox in British English
noun: either of the two occasions, six months apart, when day and night are of equal length