An equinox occurs twice a year, when the tilt of the Earth’s axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth’s equator. The term equinox can also be used in a broader sense, meaning the date when such a passage happens. The name “equinox” is derived from the Latin aequus (equal) and nox (night), because around the equinox, the night and day have approximately equal length.
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: classically, the vernal point and the autumnal point. By extension, the term equinox may denote an equinoctial point.
An equinox happens each year at two specific moments in time (rather than two whole days), when there is a location (the subsolar point) on the Earth’s equator, where the center of the Sun can be observed to be vertically overhead, occurring around March 20/21 and September 22/23 each year.
Although the word equinox is often understood to mean “equal [day and] night,” this is not strictly true. For most locations on earth, there are two distinct identifiable days per year when the length of day and night are closest to being equal; those days are referred to as the “equiluxes” to distinguish them from the equinoxes. Equinoxes are points in time, but equiluxes are days. By convention, equiluxes are the days where sunrise and sunset are closest to being exactly 12 hours apart.
The seasons of the Earth are caused by the fact that the rotational axis of the Earth is not perpendicular to its orbital plane. The axis of the Earth is tilted at an angle of about 23.44 degrees from its orbital plane. This tilt is called the axial tilt. As a consequence, for approximately half of the year (i.e. from around March 20 to around September 22), the northern hemisphere tips toward the Sun, with the maximum amount occurring on about June 21, while for the other half of the year, the southern hemisphere has this phenomenon, with the maximum around December 21. The two instants when the Sun is directly overhead at the Equator are the equinoxes. Also at that moment, both the North Pole and the South Pole of the Earth are just on the terminator, and hence day and night are equally divided between the northern and southern hemispheres. At the equinoxes, the rate of change for the length of daylight and night-time is the greatest.