Solitude

 was a mountain on Gallifrey.

According to the Tenth Doctor, the Citadel of the Time Lords stood in the mountains of Solace and Solitude on the Continent of Wild Endeavour. (TV: The Sound of Drums)

In the Old Times, Presus fought Death's Messenger in the foothills of the mountains of Solace and Solitude for three days and nights. (PROSE: Blind Fury)

The Eighth Doctor thought young Time Lords such as Veeda and Norvid should be scaling the mountains of Solace and Solitude, rather than fighting in the Last Great Time War. (AUDIO: The Conscript)

One source stated that the mountains were destroyed along with the rest of Gallifrey at the end of the Time War, (PROSE: Doctor Who: The Encyclopedia) whilst another account established that the planet had been saved and relocated to a pocket universe by all incarnations of the Doctor. (TV: The Day of the Doctor)

Behind the scenes
The Dalek Handbook claimed that the Solace and Solitude burnt during the Daleks' assault on Gallifrey, the valley between them a pit of fire.