The sidereal year (sy) is a unit of measurement of time.
Definition of the sidereal year
A sidereal year is the time taken by the Earth to orbit the Sun once with respect to the fixed stars. It is slightly longer than the tropical year by about 20 minutes because of Earth's axial precession.
Value
We use the value for the sidereal year at J2000: 365.256363004 days of 86400 seconds each.
Paris Observatory IERS: Useful Constants
Value in terms of the SI
One sidereal year is equivalent to 31558149.7635456 seconds.
Some Facts
The precession of Earth's axis of rotation follows a cycle of 26000 years and has some interesting consequences.
Polaris, commonly called the North Star, will not always mark the north celestial pole, it will slowly drift away over the years and be replaced by another star when one bright enough will come into alignment with the north celestial pole.
The seasons in the north and south hemispheres are also affected since they are directly linked to the orientation of Earth's axis of rotation. If we were to follow a calendar that would not take axial precession into account, after 13000 years winter and summer would be inverted.