Pocket universe

According to the Eleventh Doctor a pocket universe was a "distorted echo of our own" universe. He indicated such realities occurred spontaneously but that they "never last[ed] for long". The Doctor also emphatically told Clara that a pocket universe was not a parallel universe.

A dangerous species from the Dark Times was closed by other races in a pocket universe called the Ringpull. They were released by the Fifth Doctor and Turlough. (AUDIO: Ringpullworld).

Hila Tacorien was once trapped in a pocket universe, and was only brought back to the normal universe thanks to the work of Emma Grayling, an empathic psychic who was able to, as the Doctor put it, act "like a lantern, shining across the dimensions, guiding her home".

Because of their short lives, pocket universes were highly dangerous to the TARDIS, which could only survive for a few seconds before being inexorably trapped there. (TV: Hide)

Notes

Despite the Eleventh Doctor's statement, there are examples of pocket universes that did last for eons, such as the Ringpull. However, the Ringpull's longer existence might be connected to the fact that it was artificially created.