Equinox (celestial coordinates)
Sidereal time is the hour angle of the equinox.  However, there are two types: if the mean equinox is used (that which only includes precession), it is called mean sidereal time; if the true equinox is used (the actual location of the equinox at a given instant), it is called apparent sidereal time. The difference between these two is known as the equation of the equinoxes, and is counted in the Astronomical Almanac.