Equinox has two separate but related meanings.
It refers two the occurrences in the year when day and night are the same length. There is a moment when the tilt of the Earth is such that neither pole is toward the Sun, and every part of Earth between the poles is positioned to have a 12-hour day and a 12-hour night.
The two equinoxes are termed:
This is the same as the exact moment when the Sun is directly over the equator at the spot on Earth where the Solar Time is noon.
The term Equinox is also used to refer to an absolute time upon which to base celestial coordinates that depend upon the Equinox. J2000.0 refers to the time January 1, 2000, 11:58:55:816 UTC, which is currently widely used as the Epoch and Equinox (aka J2000.0 Equinox) for specifying celestial coordinates.
Both the Ecliptic Coordinate System and the Equatorial Coordinate System depend upon the time of Equinox, specifically the Vernal Point, which is the direction the Sun is seen at the time of the Vernal Equinox, which is used as the basis for Ecliptic longitude in the Ecliptic Coordinate System of Right Ascension in the Equatorial Coordinate System.
Coordinates dependent upon the Equinox drift because the Equinox drifts with the various Precessions in Earth's axis and orbit. At any specific absolute time, Earth is positioned so the Vernal Equinox is at a particular point.
Celestial Coordinate System
Ecliptic Coordinate System
Equatorial Coordinate System
Right Ascension (RA)