Resurrection of the Master

The resurrection of the Master was an act that allowed the Master to be rescued from their gaseous state, which was a result of the destruction of the possessed deceased vagrant's body, thus resulting in the Master being reborn into a bald-headed new body.

History
In one account of the end of the Last Great Time War, the TARDIS had long "suffer[ed]" since the High Council's resurrection of the Master before it eventually took a new shape following the fall of Gallifrey. (PROSE: Doctor Who and the Time War)

By another account, the Master's resurrection was carried out by his own past and future selves following his death at the hands of the Ravenous. The other Masters had made a deal with the Celestial Intervention Agency to accomplish this, in return for them handing over Artron's technology. (AUDIO: Day of the Master)

Legacy
The Saxon Master, whilst speaking with the Tenth Doctor, recalled that the Time Lords had only resurrected him because they knew that he would be the perfect warrior for a time war. (TV: The Sound of Drums)