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)

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.