eq ‧ ui ‧ nox /ˈiːkwənɒks, ˈiːkwɪnɒks, ˈe- $ -nɑːks/ BrE AmE noun [countable]
[ Date: 1300-1400 ; Language: French ; Origin: équinoxe , from Latin aequinoctium , from aequi- ( EQUI- ) + nox 'night' ]
one of the two times in a year when night and day are of equal length ⇨ solstice :
the spring equinox