Webster's English Dictionary

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equinox

equi.nox \'e--kw*-.na:ks, 'ek-w*-\ n [ME, fr. MF or ML; MF equinoxe, fr. ML 
   equinoxium, alter. of L a]equinoctium, fr. aequi- equi- + noct-, nox night 
   - more at NIGHT 1: either of the two times each year when the sun crosses 
   the equator and d ay and night are everywhere of equal length, being about 
   March 21 and September 23 2: either of the two points on the celestial 
   sphere where the celestial equ ator intersects the ecliptic