noun

• One of the two occasions in the year when the length of the day and night are equal, which occurs when the apparent path of the Sun (the ecliptic) intersects with the equatorial plane of the Earth; this happens on a day between March 19 and 21 (spring in the Northern Hemisphere, and autumn in the Southern Hemisphere), and on another day between September 21 and 24 (autumn in the Northern Hemisphere, and spring in the Southern Hemisphere); hence, the exact time when the intersection occurs.
• (also figuratively) The circumstance of a twenty-four hour time period having the day and night of equal length.
• (astronomy) One of the two points in space where the apparent path of the Sun intersects with the equatorial plane of the Earth.
• (obsolete)
• (rare) A gale (“very strong wind”) once thought to occur more frequently around the time of an equinox (sense 1), now known to be a misconception; an equinoctial gale.
• (astronomy) A celestial equator (“great circle on the celestial sphere, coincident with the plane of the Earth's equator (the equatorial plane)”); also, the Earth's equator.