Bubble universe

A bubble universe was a type of tiny universe, usually attached to the main universe via a bubble universe rift. Bubble universes could be created by actions in the main universe, and sealed off from it entirely.

In violation of most other accounts of the nature of the multiverse, (TV: Army of Ghosts, et. al) The Book of the War wrote off the idea of "parallel universes" being real and instead claimed different universes were merely be "bubbles" consisting of energy and matter. It stated these "bubbles" were "cut off" from each other by un-space. (PROSE: The Book of the War)

Known bubble universes
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)

One bubble universe on the edge of N-Space was home to a sentient planetoid known as House, who lured Time Lords from the main universe through the bubble universe rift so it could feed off the artron energy from their TARDISes. After the Eleventh Doctor was lured there by a distress signal left by the Ninth Corsair, the bubble universe reached absolute zero while House was inside of the Doctor's TARDIS. (TV: The Doctor's Wife)

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

Other references
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)