Definition
The sidereal period (or sidereal year for Earth) is the time for a body to complete one full revolution around the Sun as measured against the distant stars. Earth's sidereal year is about 365.256 days.
Kepler's third law
For planets, the sidereal period P (in years) and semi-major axis a (in AU) satisfy P² = a³. This links orbital size and speed across the solar system.
