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 himself in the normal universe fleetingly. His Graske also had access to Limbo, and could materialise and dematerialise in Limbo or reality at will.
The Trickster sent Sarah Jane Smith to Limbo when she was removed from time. Maria Jackson was also sent here. (TV: Whatever Happened to Sarah Jane?) He later put Sarah Jane and the Tenth Doctor, Luke, Rani, Clyde and K9 in Limbo. He put Sarah Jane in a time differently looped than the others. (TV: The Wedding of Sarah Jane Smith)
Other uses of the term
The Fourth Doctor claimed that the first five segments of the Key to Time were being kept 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. (TV: The Armageddon Factor)