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.
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.