equinoxcelestial sphere showing the positions of the autumnal and vernal equinoxese·qui·nox (??kw?-n?ks?, ?k?w?-)n.1. Either of two points on the celestial sphere at which the ecliptic intersects the celestial equator.2. Either of the two times during a year when the sun crosses the celestial equator and when the length of day and night are approximately equal; the vernal equinox or the autumnal equinox.[Middle English, from Old French equinoxe, from Medieval Latin aequinoxium, from Latin aequinoctium : aequi-, equi- + nox, noct-, night; see nekw-t- in Indo-European roots.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.equinox (?i?kw??n?ks; ??kw??n?ks) n1. (Astronomy) either of the two occasions, six months apart, when day and night are of equal length. See vernal equinox, autumnal equinox2. (Astronomy) another name for equinoctial point[C14: from Medieval Latin equinoxium, changed from Latin aequinoctium, from aequi- equi- + nox night]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014e?qui?nox (?i kw??n?ks, ??k w?-) n. 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) and Sept. 22 (autumnal equinox). [1350?1400; Middle English