Vernal Equinox

Equinox can have two meanings:
The moment when the Sun is positioned directly over the Earth's equator and, by extension, the apparent position of the Sun at that moment—see below.
The time at which the vernal point, celestial equator, and other such elements are taken to be used in the definition of a celestial coordinate system—see Equinox (celestial coordinates)
An equinox in astronomy is that moment 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 23 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.