A moon (also known as a natural satellite) was an astronomical object found in orbit around a planet.
Some moons, like Voga, were planetoids brought into orbit around a planet by the planet's gravity, while others were the result of a collision between objects during the formation of a solar system. (TV: Revenge of the Cybermen)
The Eleventh Doctor once offered to take Amy Pond and Rory Williams to a moon that was actually made of honey for their honeymoon. He later corrected himself, saying it wasn't made of actual honey, and it wasn't actually a moon. It was in fact technically alive "and a bit carnivorous, but there are some lovely views." (TV: A Christmas Carol)