Pocket Dimension

Another Dimension that is not so "other". Pocket Dimensions are spaces that are too small or too easily accessible to be truly considered a separate dimension and are referred to as a small extra pocket of space that is attached to our own. Much like an actual pocket, they are often used for some extra space where you can get things Bigger on the Inside. A Speculative Fiction favourite, the uses are plentifold. Storage for a Bag of Holding, hiding places, transportation, an explanation for physics defying superpower: a Pocket Dimension can do them all. Can't make julienne fries though.

Also can serve as a (sometimes unstable) Small Secluded World with its own ecosystem and lifeforms. This is a quite handy place for keeping some nasty lurksome monsters; it lets them be very alien and make intermittent contact.

See also Just One Second Out of Sync which is often exactly the same but explained in terms of a temporal fashion than a spatial one. Not to be confused with a Palm OS game of the same name.

Anime and Manga

 * In addition to Doraemon's Bag of Holding pouch on his stomach, his time machine is also parked in a pocket dimension accessed from Nobita's desk.
 * Negima has a number of Pocket Dimensions around; Kaede's magic cape contains one, and Mana uses one to store her ammunition.
 * Also there are Evangeline's castles, Theodora's diorama sphere and Evangeline's Magia Erebea scroll.
 * In Naruto Madara can open a gate to his (presumably) own pocket dimension with his Sharingan, which is a black void full of white boxes.
 * Gluttony of Fullmetal Alchemist has a Pocket Dimension in his stomach.
 * Blueno's Devil Fruit from One Piece allows him to access one by making a door in the air itself.
 * Star Driver has Zero Time.
 * In Saint Seiya Gemini Saga 's trademark attack "Another Dimension" warps you into one of these. The really overblown delivery has reached massive memetic mutation status.

Comic Books

 * The Marvel Comics Heroes Reborn universe was explicitly called a Pocket Dimension, or Pocket Universe, which Franklin Richards literally carried in his pocket.
 * Another one can be accessed through use of the Soul Gem.
 * There's also the Alternate Universe of Marvel1602 which was split off from the main Marvel Universe to prevent all of reality being destroyed and placed into a jewel guarded by Uatu.
 * Archie Comics' Sonic the Hedgehog series explicitly explains Zones as 'pocket dimensions' via editor notes. This is most apparent in the case of the Special Zone (equivalent of Special Stages from the games). This is made confusing by the fact that areas within the main dimension are also referred to as 'Zones' to comply with the games' stage names eg. Green Hill Zone, Lava Reef Zone etc.
 * An apparently commonplace practice for Green Lantern officers is to place their lantern-shaped power battery (which they must use to recharge their rings periodically) inside a pocket dimension, so as to have easy access to it in the field. Kyle Rayner keeps his in his apartment, which has more than once worried him while off on another planet, since he might not have enough juice in the ring to get home.
 * The Time Trapper created a pocket universe which events from the Pre-Crisis/Silver Age Superboy stories took place. Anytime the Legion of Super-Heroes travel back to meet their inspiration Superboy, the Time Trapper diverts them to there. The only inhabitable planet is Earth (and so was Krypton). It has become a dead world after Zod, Faora, and Quex-Ul killed everyone on Earth, following Superboy's death by the Time Trapper. It's apparently destroyed during Zero Hour.

Fan Works

 * In With Strings Attached, the original bodies of the four, and later their cloned bodies, are stored in a pocket dimension, a private stasis pocket.
 * The Varga essentially creates one from scratch in order to establish a domain in which The Family is pre-eminent, for the purpose of abusing a loophole in Dragon's programming (for her benefit) in the Worm/Luna Varga crossover Taylor Varga.

Literature

 * The Fair Folk in Lords and Ladies are explained to live in a parasite dimension that just sort of floats around our own, waiting for times when the Theory of Narrative Causality itself allows them to burst through.
 * Death's Domain exists in a similar world.
 * nearly every anthropomorphic personification has been shown to have or use one of these, from the Kaos, to the tooth fairy.
 * In Dark Lord of Derkholm, Mara's specialty is making these.
 * The Room Of Requirement in Harry Potter
 * Not to mention the Weasleys' tent.
 * The future human civilization of Walter Jon Williams's 2010 novel Implied Spaces uses pocket dimensions maintained by vast post-human artificial intelligences as living space.
 * In Michael Moorcock's Elric of Melnibone stories, the Half Worlds where the Beast Lords live.
 * Fablehaven has the transdimensional knapsack.
 * The Other World in Coraline. She even comments about it : "Small world", indicating it's not really another dimension.

Live-Action TV

 * Doctor Who (classic series) had a trilogy of stories near Tom Baker's departure set in E-Space which is described as "a smaller universe existing alongside the prime universe". Click for more.
 * The last E-Space story, "Warriors Gate", is set inside an even smaller pocket dimension—small enough to cross on foot—on the border between E-Space and N-Space.
 * The TARDIS interior itself is often described this way, along with many other things bigger on the inside.
 * The world of the Celestial Toymaker is described as this.
 * The bubble universe of House also qualifies.
 * In Land of the Lost, they can stand on a hilltop and look through binoculars... and see the backs of their own heads.
 * In Are You Afraid of the Dark?: "The Tale of the Super Specs", the protagonists end up trapped in one when the Alternate Universe overtakes the normal universe's space. In "The Tale of the Doll Maker", Susan is trapped in Creepy Doll form in a dollhouse accessed by a one-way portal in the attic of the normal-sized house. In "The Tale of the Pinball Wizard", Ross ends up trapped in a replica of the mall inside the pinball machine in Mr. Olson's shop.

Tabletop Games

 * While the cosmology of the Dungeons & Dragons universe changes over the year, at various points you can be rest assured this trope appears and plays a major part. Various "planes" exist not as full world but as smaller "demi-plane" realms which can occasionally collide with ours and unload some XP filled monsters. One of the games most iconic elements, the Bag of Holding, is explained as being a hole which leads to another plane thus allowing it to be Bigger on the Inside. So you can get a pocket in your pocket.
 * Baldur's Gate II, a video game based on the Dungeons & Dragons Forgotten Realms setting has one level in which you can follow a kidnapped band of actors into a pocket dimension where slaves with Shock Collars are ruled over by demons who live in O Ring Orifices. What Do You Mean It Wasn't Made on Drugs??
 * The Throne Of Bhaal expansion/'sequel' gave the PC their own pocket plane, a lovely little pied-Ã -terre situated just off Bhaal's layer of the Abyss and within easy commuting distance of anywhere.
 * The spell "Rope Trick" created an extra-dimensional space that you could crawl up to (and into) using a rope.
 * Adventure WG6 Isle of the Ape had a moderate sized "non-dimensional" space in which PCs could be trapped if they weren't careful.
 * Adventure T1-4 Temple of Elemental Evil had elemental nodes (partial planes) that were about 5 miles across.
 * Adventure I12 Egg of the Phoenix had a partial plane called Sepulchre, also about 5 miles across.
 * Adventures EX1 and EX2 take place on a partial plane of limited (unspecified) size.
 * The Basic/Expert/etc D&D adventure "Skarda's Mirror" features a dimension accessible through the titular magic item. The bandit warlord Skarda uses to raid cities by filling it with soldiers and then having it smuggled, sold or given to the occupants.
 * An Epic-level spell allows you to build your very own Pocket Dimension. It can potentially grow to infinite size.
 * In the Old World of Darkness, pretty much every species of playable monster had a way of creating one of these with enough magical energy around leylines, calling them invariably: Dragon Nests, Nodes, Caerns, Freeholds, and Haunts.
 * Classic Traveller Adventure 12 Secret of the Ancients. 300,000 years ago the Ancient known as Grandfather used ultra-advanced technology to pinch off three solar systems from the rest of the universe for his private use. Other Ancients used the same technology to remove smaller areas for various purposes.
 * Hackmaster 1E. Bags of Endless Storage and Bags of Hefty Storage Capacity access an extra-dimensional area called "Bagworld".
 * Exalted has the aptly-named Elsewhere. Things stored in it are "safe enough", in that it can't be taken back except by the person storing it, and things stored by different person won't interact with each others. It has been used to store weapons, souls (in an And I Must Scream fashion), and a frickin' Primordial that is a world in itself.
 * You can become one, if you're a Green Sun Prince. It's a world defined as you like it, you can invite others to live in it, and it grows as you feed it Essence.

Video Games

 * Xen in Half Life looks to be some sort of pocket-dimension/InnBetweenTheWorlds, filled with an alien atmosphere and what appear to be broken fragments of the Nihilath's shattered homeworld, inhabited by weird xenofauna.
 * Wolfenstein's Black Sun Dimension also looks like a very small, isolated spherical volume of space. At its center is the Black Sun, an inexhaustible source of strange energy. The best guess is that the Black Sun is the only thing keeping that place from collapsing on itself in a Big Crunch.
 * Prey's  takes place in one of these. It's origin and purpose are not clear, but judging by the mining explosives found there, it might be used by the aliens for storing extra-large asteroids prior to mining.
 * In City of Heroes there is the interdimensional dance club Pocket D, a neutral zone where heroes and villains can get together but are incapable of attacking one another.
 * At the far end of Endgame: Singularity's tech tree is the ability to build reality bubbles where you can carry out experiments that would otherwise run the risk of destroying the universe.
 * In Master of Orion II, the Antarans were banished by the Orions to a pocket dimension the size of a single star system. Their escape is the premise of the game, and you can invade Antares yourself if you build a Dimensional Gate. If you don't want to deal with them, you can turn the "Antarans Attack" option off.
 * The magical world of Gensokyo was sealed off from the rest of the world when people decided that they didn't want to be bothered with magical creatures anymore. Since then there's the occasional human who wanders in by accident / gets dragged in by Yukari, but they either settle down in the Human Village or get eaten by one of many Youkai.
 * Super Mario 64 has this unintentionally, in the form of the "Black Room of Death", a glitch room that is too large to exist where it is.
 * Dimentio's "Dimension D" in Super Paper Mario, which multiplies his strength by 256(though this effect is also given to anyone else who enters it).
 * The "Sharpened Shield" Shield World in Halo: Ghosts of Onyx. Furthermore, Forerunner Slipspace Pods also trap their occupants in pocket dimensions, so some characters in Ghosts of Onyx ended up locked in a pocket dimension within a pocket dimension.
 * Divine Divinity has a gobelin living inside a cristal ball carried by another gobelin. When the hero looks into the ball, he is sucked inside it and reappears in a small garden near a mansion where the seeked gobelin lives. There a quite a share of uncanny things there too, such has sudden bursts of rain, a war between bees and wasps (with each side asking you for support and none being the "good" side) and a lot of closed recipients with a handful of keys lying around.
 * In World of Warcraft, a quest involving wizard of the Kirin Tor, an entire city is located in a pocket dimension which the player must breach and kill all the wizards.
 * There are Pocket Dimension portals in RuneScape... Used for holding PC houses.
 * In Oblivion, you go inside a painting for the quest "A Brush with Death." may count, as well.

Web Original

 * Genius: The Transgression has Bardos—pocket worlds made of concepts disproved by science. The more prominent ones include an alien-inhabited Mars, the Hollow World (home to dinosaurs and cavemen and Nazis), and the Seattle of Tomorrow, which Lemuria tried to bring into this world with disastrous consequences.

Western Animation

 * In ReBoot the Game Cubes function like this. Once they land the area inside is completely replaced and sealed off from the outside world. They also move between systems and can be used for random transport.

Real Life

 * The Internet.