Pocket universe

According to the Eleventh Doctor, a pocket universe or pocket dimension was a "distorted echo of our own" universe. He indicated such realities occurred spontaneously but that they "never last[ed] for long". However, there were also examples of artificially created and longer lasting pocket universes. The Doctor also emphatically told Clara that a pocket universe was not a parallel universe. (TV: Hide) Nonetheless, the planet Gallifrey and the people on it were sent into was referred to by the War Doctor as being a "single moment in time, held in a parallel pocket universe. Like a painting!" (TV: The Day of the Doctor) 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 as the universe collapsed back into the quantum foam. (TV: Hide)

While the terms "pocket universe" and "pocket dimension" may have been interchangeable, Albert Marsden commented that he was not trapped in a whole universe, nor a whole planet, explaining that his "pocket dimension"/time loop merely stretched for five miles and ten days before restarting. (AUDIO: Protect and Survive)

Examples
Ecto-Space was a large pocket universe, containing numerous star systems and a Time Vortex of its own. It had subtly different physical laws from the "main" universe, but was otherwise stable. (AUDIO: The Choice)

The Divergent Universe was a pocket universe that had no concept of time. Rassilon considered it the most "significant" of the universes he kept inside his dungeon on Gallifrey. (AUDIO: Zagreus)

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)

The Galacti-Bank vault utilised dimensional transcendentalism. The contents of the vault were contained in a pocket universe which could only be accessed by using the correct combination, which changed four times per day. (AUDIO: The Selachian Gambit)

The Seventh Doctor trapped two Elder Gods in a pocket dimension that comprised of just five miles of Britain after a nuclear war. (AUDIO: Protect and Survive)

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". This pocket universe contained a large island seemingly floating in space, on which was a misty forest. Time also ran much slower in this pocket universe than in the prime universe, to the point that 1 second in the pocket universe was equivalent to 100,000 years in the Doctor's universe. (TV: Hide)

The Celestial Toyroom (TV: The Celestial Toymaker) and the Land of Fiction were thought to be pocket universes. (TV: The Mind Robber)

In order to save the planet from the Daleks, "all thirteen" incarnations of the Doctor teamed up together to freeze Gallifrey in time and place it in a "single moment in time, held in a parallel pocket universe." While they were unsure if they succeeded, an encounter with the Curator indicated to the Eleventh Doctor that this was in fact successful. According to the Eleventh General and the Doctors, the calculations necessary would take several hundred years so the Doctor worked on them for all of his various lives. (TV: The Day of the Doctor) A doorway to this pocket universe in the form of a crack in time appeared in the town of Christmas on Trenzalore. It was through there that the Time Lords sent the message "Doctor Who", which attracted the Eleventh Doctor as well as various races across the universe, and, 900 years later, with the Doctor dying from old age at the end of his final incarnation, energy for a new regeneration cycle. (TV: The Time of the Doctor) Having confirmed that they'd found N-Space, the Time Lords returned Gallifrey to it, (PROSE: A Brief History of Time Lords) at the extreme of time continuum for its own protection. When the Doctor returned, he purposefully didn't try to find out how this had been accomplished, knowing that it would have made the other Time Lords "feel clever". (TV: Heaven Sent, Hell Bent)

The Vess kept a weapons factory in a pocket dimension, powered by energy drawn from the Big Bang. (AUDIO: The Light at the End)

Shadow-Space was an artificially created pocket dimension that stored the consciousness of intergalactic starship crew to protect them during warp flights. (AUDIO: Masquerade)

Having heard the Question, and fearing that the Time Lords would revert the universe to what it was before the Last Great Time War, the Voord, having grown more powerful during the conflict, harvested discarded Dalek technology to hide their civilization in a very small pocket universe. Through the efforts of the Tenth, Eleventh and Twelfth Doctors, the Voord were forced to collapse their reality and ultimately revert themselves to their weaker pre-Time War state. (COMIC: Four Doctors)