Synonym: Autumnal Equinox
The point at which the Sun crosses the celestial equator from North to South along its apparent path along the ecliptic, which occurs around the 23rd September when the number of daylight hours is the same at all places on the planet. It is exactly opposite the point of the spring equinox.
The earth's precession causes both the spring equinox and the autumnal point to move backwards through the zodiac. At the time of its discovery, the autumnal point was in the constellation of Libra which is why it is also sometimes referred to as the first point of Libra. Due to the precession it is now located in the constellation of Virgo.
This is irrelevant in Western astrology which is based on the tropical zodiac in which the autumnal point always corresponds to 0 degree Libra.