Point of Stillness

 was a place where the Time Lords were forbidden to go. The Fourth Doctor tried to explain to Leela about it and its relation to time travel. The Time Lady Marianna was imprisoned inside the Doctor's TARDIS after attempting to find the Point of Stillness. (AUDIO: The Abandoned)

Behind the scenes
The Point of Stillness previously appeared in Nigel Fairs' Audio Adventures in Time & Space story Guests for the Night, featuring the Professor and Ace.