Definitions for "Sidereal time"
Measurement of time based on the position of the stars, rather than the Sun. While it takes the Earth 23 hours, 56 minutes, 4.091 seconds to rotate once on its axis relative to the Sun, it takes exactly 24 hours for it to rotate once with respect to the stars. Therefore, a solar day, which is measured with respect to the Sun, is slightly shorter than a sidereal day, which is measured with respect to the stars. The difference between solar and sidereal time is due to the Earth's motion - as we move along our orbit around the Sun, the relative position of the Sun compared to the background stars changes, so the clocks shift by just a little bit each day. Astronomers use this time keeping method when they are making observations, because the current sidereal time equals the right ascension coordinate that is crossing the meridian at that moment. This makes it easy to compare the right ascension of an object and the local sidereal time to figure out the optimum time to observe an object.
Time measured with respect to the stars as opposed to the sun. 1 mean sidereal day = 23h 56m 4.09053s of mean solar time. (The Astronomical Almanac 1996 p. B6)
the measure of time defined by the apparent diurnal motion of the catalogue equinox; hence a measure of the rotation of the Earth with respect to the stars rather than the Sun.