The time for one complete orbit of the Moon around the Earth relative to the distant stars.
Symbols
Tsyd
sidereal period — one full orbit relative to the stars
[d]
Where it comes from
Shorter than the synodic month, because it does not have to "catch up" with the Earth's motion around the Sun. It equals the Moon's rotation period (synchronous rotation — which is why we always see the same side).
When to use it
When you compare the orbital period with the period of the phases, or explain synchronous rotation.