1. A reference to the stars or constellations: "sidereal bodies".
2. Relating to the stars, especially measured with reference to the apparent motion of the stars.
3. Divisions of time; determined by daily motion of the stars: "sidereal time".

In astronomy, relating to the period of time based on the apparent rotation of the stars, and therefore equivalent to the rotation of the body from which the observation is made. On earth, a sidereal year is 365.256 times the sidereal day of 23 hours, 56 minutes and 4 seconds.

The time it takes for a planet or satellite to make one complete circuit of its orbit (360°) relative to the stars: "The Earth's sidereal period, or sidereal year, is equal to 365.2564 mean solar days."

The measure of time defined by the apparent diurnal motion of the catalog equinox; hence, a measure of the rotation of the earth with respect to the stars rather than the sun.