- Either of two points on the celestial sphere at which the ecliptic intersects the celestial equator.
- 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.
- 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 <em>vernal</em>, which falls in the spring, namely, on the 21st of March according to the Gregorian calendar, and the <em>autumnal</em>, which falls in the autumn, namely, on the 22d of September. The term <em>equinox</em> is also loosely applied to the <em>equinoctial points</em> (which see, under <internalXref urlencoded="equinoctial">equinoctial</internalXref>).
- An equinoctial gale or storm; an equinoctial.
- Anything equal; an equal measure.
- 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 <xref urlencoded="autumnal">autumnal</xref> and <xref urlencoded="vernal">vernal</xref>.
- Equinoctial wind or storm.
- The <xref>intersection</xref> of the <xref>ecliptic</xref> (apparent path of the sun) with the <xref>celestial</xref> <xref>equator</xref>.
- (astronomy) either of the two celestial points at which the celestial equator intersects the ecliptic
- 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