Moon

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)

Sol
By the year 200,000, Earth was orbited by five moons. (TV: The Long Game)

The Third Doctor sang of Martian moons. (TV: Inferno)

Other moons
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)

The Shadow Proclamation tended to the matter of "suicide moons". (TV: The Magician's Apprentice)

Other references
"Moons of madness" was a curse sworn by the Captain. (TV: The Pirate Planet)

"By the moons" was an oath sworn by the Ice Warriors and those who did them honour, such as Clara Oswald and the Twelfth Doctor. (TV: Cold War, Empress of Mars)

Luna