1. Time that is measured by the stars rather than by the sun: Sidereal time is based on the sidereal day, which is the unit of measurement.

The sidereal time is a unit of time used in astronomy, equal to the period of time in which the Earth makes one rotation relative to the stars.
2. The time taken between successive meridian passages of the same star (23 hours, 56 minutes, 4.091 seconds): The sidereal day is about four minutes shorter than the twenty-four hours of the "solar day".