The Zero Room was an important room in the Doctor's TARDIS.
Zero Rooms were used by the Time Lords in times of difficult regeneration or to rapidly heal. There was, for instance, a renowned polygonal Zero Room under the Junior Senate Block on Gallifrey. (TV: Castrovalva) They were cut off from the random electrical and radiological influences of the rest of the universe. They were ideal for neurological healing. (AUDIO: The Invasion of E-Space) They had a tranquil atmosphere. As the Fifth Doctor demonstrated to his companions, some physical laws did not hold as strongly in a Zero Room as in the rest of the universe. For instance, he could levitate in the TARDIS's Zero Room at will. The Zero Room also enhanced his telepathic abilities so he could speak to his companions while resting. (TV: Castrovalva)
The Zero Room was used by the Fifth Doctor following the regeneration of his fourth incarnation. In order to generate enough power to break free of Event One, part of the structure of the TARDIS needed to be ejected. Unfortunately, the Zero Room was one of the areas destroyed. Using the remaining walls of the room, Nyssa was able to construct the Zero Cabinet. It had the same properties as the room, which allowed the Doctor to continue recuperating while he was within it. Nyssa and Tegan found the cabinet light enough to carry, in part because the Doctor used levitation to negate his own weight. However, if the Doctor did not concentrate on keeping himself levitated, the cabinet became too heavy to carry, due to his weight, and would plummet to the ground. (TV: Castrovalva)
Later, the Zero Room was reconstructed in the TARDIS. (AUDIO: Renaissance of the Daleks) The Sixth Doctor used it to stabilise Charley Pollard when she was sick. (AUDIO: Patient Zero) He also used it to lessen the effects of time going backwards, when trying to detach a parasitic being from the TARDIS. (COMIC: Funhouse) The Seventh Doctor kept a reconstructed Zero Room (PROSE: Deceit)
Soon after the TARDIS became infected by an organic material from Tír na n-Óg, (PROSE: Cat's Cradle: Witch Mark) the Doctor began adapting the Zero Room into another console room, disconnected from the universe, so he could escape the effects. This control room/Zero Room was lost when the Doctor ejected it from the TARDIS in an attempt to destroy the sentient computer, Pool. (PROSE: Deceit)
The Twelfth Doctor jettisoned the Zero Room, allowing the Celestial Toymaker to use it as his new Toyroom. He remarked afterwards to Clara Oswald that he would need a new Zero Room. (COMIC: Relative Dimensions)