Noun
synchronous orbit (plural synchronous orbits)
A satellite orbit where the orbital period is equal to, or multiples of, the Earth’s rotational period; i.e. making one, two, three, etc., orbits in a 24-hour period. Examples include geosynchronous (period equals Earth's rotation), semisynchronous (two orbits per day); and geostationary (geosynchronous orbit where satellite maintains a fixed position on the equator).
Orbital decay can occur due to tidal forces for objects below the synchronous orbit for the body they're orbiting. Source: Internet
This is because the smaller body becomes tidally locked faster, and by the time a synchronous orbit is achieved, it has had a locked synchronous rotation for a long time already. Source: Internet
With an altitude of convert, Phobos orbits Mars below the synchronous orbit radius, meaning that it moves around Mars faster than Mars itself rotates. Source: Internet