Poosh

Poosh was the name of either a moon that disappeared, or the planet around which an unnamed moon orbited. Efforts to find the moon were unsuccessful, at least by the time the Tenth Doctor visited the planet of Midnight with Donna. Dee Dee Blasco had become a research assistant to Professor Hobbes on the strength of a paper she had written about Poosh. The timing and circumstances of Poosh's disappearance were not revealed to the Doctor. (DW: Midnight)

The Doctor discovered that the Daleks had stolen this planetary body out of time and space and hidden it in their time pocket in the Medusa Cascade in order to use it to power the Reality Bomb. (DW: The Stolen Earth) The Doctor with the help of his former companions helped bring this planet back along with all the 26 others the Daleks had taken. This particular one was returned by Donna Noble who along with the Doctor and the New Doctor managed to return all of the planets except Earth which was towed home. (DW: Journey's End)

Behind the Scenes
The script for Midnight is slightly vague about whether "Poosh" names the moon or the planet around which the moon orbits. Because the phrase "the lost moon of Poosh" is employed, it is possible to think of Poosh as the planet, and the moon being unnamed. However, the Doctor proposes a toast to "Poosh" and Dee Dee accepts the toast without correction. In the context of the conversation - during which the Doctor suggests that Dee Dee's contribution to science might be finding the lost moon - Poosh is more likely the moon's name, rather than the planet's. This seems to have been confirmed by the holographic image of the planetary body seen in The Stolen Earth. However, the script to this later story muddies the waters further by calling Poosh "one of the 27 planets" stolen by the Daleks.