A moon is a planetary body that is a natural satellite of a planet. Many of the moons of the various planets in the Solar System were inhabited by human populations by the 22nd century, with the Saturnian and Jovian moons proving to be particularly popular with colonists.
Earth's moon, Luna, is sometimes referred to by the people of Earth as The Moon. The seventh Lunar city is the subject of Dave Lister's favourite song, which he frequently sings throughout the series, suggesting Lister had spent at least some time there.
A Psi-moon is a type of moon that reshapes itself to match the mental state of anyone who sets foot on its surface, employing terraforming capabilities to map the 'mindscape' of its inhabitant. Having acquired a 'master', it seemingly doesn't reshape again until that individual leaves its surface. (Terrorform)