/ ˈiːkwɪnɒks; ˈek-; NAmE -nɑːks/ noun
one of the two times in the year (around 20 March and 22 September) when the sun is above the equator and day and night are of equal length :
the spring / autumn equinox
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WORD ORIGIN
late Middle English : from Old French equinoxe or Latin aequinoctium , from aequi- equal + nox , noct- night.