Mercury

 has several meanings pertinent to the Doctor Who universe and the real world. They are listed below in alphabetical order.

In-universe

 * Mercury, an chemical element.
 * Mercury swamp, swamps containing mercury.
 * Mercury, a planet in the Sol system.
 * Mercurian energy beast, natives of the planet Mercury.

Behind the scenes

 * Mercury (short story), a Short Trips short story about the planet Mercury.