Bubble universe

A bubble universe was a tiny universe attached to the main universe via a rift.

One bubble universe was home to the sentient planetoid known as House. House lured Time Lords from the main universe through the bubble universe rift so it could consume their TARDISes. The Eleventh Doctor and many other Time Lords were drawn to the bubble universe by distress hypercubes, presumably those of House's previous victims. Notably, the bubble universe also included a junkyard filled with dead TARDIS parts left behind by Time Lords lured into House's trap. The bubble universe reached absolute zero 3 hours after House left it while he was inside of the Doctor's TARDIS. (TV: The Doctor's Wife)

A bubble universe was created when the Fifth Doctor subconsciously used Block Transfer Computation to prevent Adric's death in the freighter's descent into prehistoric Earth. In this universe, Adric ruled over the giant scorpions that resided there and extended his own life in the hope of getting his revenge on the Doctor and marrying Nyssa. This universe was undone when the scorpions realised he planned to leave, thus sealing it off from the main universe, and rebelled against him. (AUDIO: The Boy That Time Forgot)

The Universe in a Cardboard Box included all the pieces needed to assemble a bubble universe out of Parablox. (PROSE: The Book of the War)

later referred to the pocket universe which Gallifrey inhabited as the Time Lords' "little bubble universe". (TV: Spyfall)

Behind the scenes
In the script for The Doctor's Wife, writer Neil Gaiman described the junkyard as "the Totter's Lane at the end of the universe." (CON: Bigger on the Inside)