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One of two times in the year (one in March and the other in September) when the length of the day and the night are equal, which occurs when the sun is directly overhead at the equator; this marks the beginning of spring in one hemisphere and autumn in the other.
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(also figuratively) The circumstance of a twenty-four hour time period having the day and night of equal length.
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(astronomy) One of the two points in space where the apparent path of the Sun intersects with the equatorial plane of the Earth.
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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.
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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.