Crucible

The Crucible is a spaceship used by Davros and the Daleks which existed within the Medusa Cascade. It appeared as a massive spherical space station the size of a planet with numerous docking ports outside it.

During the War in the Medusa Cascade when the New Dalek Empire invaded the captured Earth, the Crucible was at 90% efficiency. (DW: "The Stolen Earth"). Human prisoners were taken for an unknown variety of testing during the Dalek invasion of Earth in 2009. The Crucible was to be the staging post for the deployment of a Reality bomb that would have destroyed all of the universe except for the inhabitants of the station allowing the Daleks to be the sole survivors of existence. (DW: Journey's End)

It should not be confused with the Cruciform which was mentioned by The Master. (DW: "The Sound of Drums").

Speculation
Given the old meaning of the word crucible, and comments made by the Dalek Emperor in The Parting of the Ways, the Crucible could be a tool used to extract the pure Dalek fragments from human DNA on a massive scale. This would also explain why, in Journey's End, the Daleks take prisonors instead of exterminating the whole of Earth

Behind the Scenes
The Crucible may be the station made for the Shadow Proclamation.