An equinox is the event of the Sun passing over the Earth's equator, in its annual cycle. By extension it also signifies the time at which it happens and the apparent position of the Sun at that moment.

In astronomical terms it refers to the intersection on the celestial sphere between the celestial equator and the plane of the ecliptic. The term is thereby used as a name for a specific junction on a set of spherical coordinates and also, being that these coordinates derive directly from the rotation of the earth and its inclination on its axis as it orbits the sun, a date for the arrival of the earth-sun system on the annual calendar which causes the aforesaid intersection.

The word is also used for the same event happening to other planets, and in setting up a celestial coordinate system—see Equinox (celestial coordinates).

An equinox happens each year at two specific moments in time (not a whole day) when the centre of the Sun can be observed to be directly above the Earth's equator, occurring around March 20 and September 22 each year.

More technically, at an equinox, the Sun is at one of two opposite points on the celestial sphere where the celestial equator ( i.e. , declination 0) and ecliptic intersect. These points of intersection are called equinoctial points —the vernal point and the autumnal point . By extension, the term equinox may be used to denote an equinoctial point.

On a day which has an equinox, the centre of the Sun will spend a nearly equal amount of time above and below the horizon at every location on Earth and night and day will be of nearly the same length. The word equinox derives from the Latin words aequus (equal) and nox (night). In reality, the day is longer than the night at an equinox. Commonly, the day is defined as the period that sunlight reaches the ground in the absence of local obstacles. From Earth, the Sun appears as a disc and not a single point of light; so, when the centre of the Sun is below the horizon, the upper edge is visible. Furthermore, the atmosphere refracts light; so, even when the upper limb of the Sun is below the horizon, its rays reach over the horizon to the ground. In sunrise/sunset tables, the assumed semidiameter (apparent radius) of the sun is 16 minutes of arc and the atmospheric refraction is assumed to be 34 minutes of arc. Their combination means that when the upper limb of Sun is on the visible horizon its centre is 50 minutes of arc below the geometric horizon, which is the intersection with the celestial sphere of a horizontal plane through the eye of the observer. These effects together make the day about 14 minutes longer than the night at the equator, and longer still at sites toward the poles. The real equality of day and night only happens at places far enough from the equator to have at least a seasonal difference in daylength of 7 minutes, and occurs a few days towards the winter side of each equinox.