In astronomy, a rotation period is the time an astronomical object takes to complete one revolution around its rotation axis relative to the background stars. For the Earth this is a sidereal day. It is different from a solar day, which is measured by the passage of the Sun across the local meridian.
All celestial objects spin. The rotation period differs according to whether the body in question is solid or fluid (gaseous). A solid object has one value, but a fluid object such as a star spins differently at the poles compared to the equator (Sun: about 25 days at the equator and about 35 days near the poles).