Limbo

Limbo, also known as the limbo dimensions, was a realm which physically looked like a White Void. The Trickster was generally limited to limbo, only able to manifest itself in the normal universe fleetingly. His Graske also had access to Limbo, and could materialize and dematerialize in and out at will.

The Trickster had sent Sarah Jane Smith to Limbo when she was removed from time. Maria Jackson was also sent here. (SJA: Whatever Happened to Sarah Jane?) The Trickster put Sarah Jane and the Doctor, Luke, Rani, Clyde and K9 in limbo. He put Sarah Jane in a different looped time than the others. (SJA: The Wedding of Sarah Jane Smith)

The Doctor described the first 5 segments of the Key to Time being in "a sort of limbo closet, in limbo" within his TARDIS. He told The Shadow the only way to access them was by adding the final piece. (DW: The Armageddon Factor)