equinox

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Either of two points on the celestial sphere at which the ecliptic intersects the celestial equator.
• noun 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.

from The Century Dictionary.

• noun The moment when the sun crosses the plane of the earth's equator, making the day and night everywhere of equal length (whence the name). There are two annual equinoxes, the vernal, which falls in the spring, namely, on the 21st of March according to the Gregorian calendar, and the autumnal, which falls in the autumn, namely, on the 22d of September. The term equinox is also loosely applied to the equinoctial points (which see, under equinoctial).
• noun An equinoctial gale or storm; an equinoctial.
• noun Anything equal; an equal measure.

from the GNU version of the Collaborative International Dictionary of English.

• noun The time when the sun enters one of the equinoctial points, that is, about March 21 and September 22. See Autumnal equinox, Vernal equinox, under autumnal and vernal.
• noun rare Equinoctial wind or storm.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The intersection of the ecliptic (apparent path of the sun) with the celestial equator.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (astronomy) either of the two celestial points at which the celestial equator intersects the ecliptic
• noun either of two times of the year when the sun crosses the plane of the earth's equator and day and night are of equal length