A moon is a generic name for a natural satellite that orbits a planet.
Moons may occur in all varieties concerning their number, form and size. The two moons of Mars are tiny dead planetoids, whereas the Bajoran moon Jeraddo was a planet-sized M-class world, before it was transformed into a wasteland.
Gas giants may have moons that are adaptable to human colonies. An example of this is Titan, which orbits Saturn in the Sol system.
More massive moons may also strongly influence their primary planet, as they exert gravitational forces, which can define a planet's orbital period as well as its seasons, like it is the case with Earth's moon, Luna.