Another Dimension



""Listen: there's a hell of a good universe next door; let's go.""

- E. E. Cummings

Very, very old trope, still used today. Another Dimension refers to universes that are "next" to our own, which require magic or Techno Babble to travel to and from. In theory, from our world they are in a direction other than the directions we are familiar with.

Alternate universes are often just a variety of Another Dimension. Unlike AUs, though, other dimensions don't necessarily have to resemble the "home" universe. There may be a Void Between the Worlds to go through.

Travel to and from another dimension is usually via some sort of door, vortex, portal, gate, window—the exact term depends on the story. Sometimes some kind of teleportation suffices.

Most "fairy lands" in Celtic Mythology and European fairy tales are effectively Another Dimension. It is generally accepted that if Hell and Heaven exist, they're not in the same physical plane that we are in. Ditto The Underworld.

Types of Other Dimensions:


 * Alternate Universe
 * Alternate Tooniverse
 * Beneath the Earth
 * Dark World
 * Inn Between the Worlds
 * Land of Faerie
 * Lost World
 * Magical Land
 * Mirror Universe
 * The Multiverse
 * Phantom Zone
 * Pocket Dimension
 * Subspace or Hyperspace

Anime and Manga

 * The Digital World in Digimon, as described by the human protagonists.
 * Mahou Sensei Negima:
 * The Otherworld in Yozakura Quartet
 * Muge Space in Dancougar. It is also seen in Super Robot Wars Compact, Super Robot Wars Compact 2, Super Robot Wars Alpha 3 and Super Robot Wars GC
 * This is where the aliens in Bokurano come from..
 * In Saint Seiya, these are weaponized, used by Gemini Saga and Kanon as a way of removing opponents from the battlefield without much difficulty. Phoenix Ikki also uses one at one point . In Saint Seiya: The Lost Canvas, Saga and Kanon's predecessors display similar aptitude in the art
 * Popotan: Where dwells.

Comic Books

 * This trope is nearly omnipresent in Superhero and supernatural comics. A comprehensive list would take up many, many pages.
 * Mr. Mxyzptlk, of the Superman comics, is from the 5th dimension, a world where everyone's nigh-omnipotent and there's a month called Pants.
 * Walk-In is based around this premise.
 * Doctor Strange often travels to other dimensions with typically psychadelic visual effects. His wife Clea was born in one of them.
 * Shade the Changing Man comes from a realm with very different dimensional properties.
 * A Wonder Woman villain called The Angler adopts a weapon that allows him to manipulate dimensions, giving him teleporting, Time Travel and travel of The Multiverse.
 * Marvel Comics has an Angler, too; a very minor character with only two appearances, he was radically transformed by being in Another Dimension and though he returns from it, isn't quite suited to "normal" space and tends to be in two places at once. Not to mention crazy, deformed and speaking in weird symbols that look like broken glass.
 * The old Earth-One and Earth-Two of DC Comics, now replaced by the Fifty-Two.
 * The Marvel Universe is number 616 out of thousands.
 * Zenith has not only the traditional Alternate Universe setup, but a dimension outside of spece and time which the Lloigor call home.
 * Although the Fantastic Four often visit the Negative Zone, it's their Ultimate Marvel counterparts who actually use it as part of their Super-Hero Origin. And they use the nature of other dimensions as a.
 * This is also Marvel's favourite Hand Wave whenever something requires physics-breaking power; the Hulk's extra mass is taken from one, Nightcrawler travels through one when teleporting, and Cyclops gets his eyebeams from one where relativity works differently.

Fan Works

 * C'hou in With Strings Attached is explicitly set in another dimension; the other planets that the four visit are also in other dimensions. In fact, “universe” and “dimension” are synonymous in this work.
 * Termina in The Blue Blur of Termina is this to Sonic's Earth, accessible only via a portal deep within the jungles of Adabat.
 * Ranma in the Mega Crossover Ranma and Akane: A Love Story has a pendant that lets him/her visit different worlds and timelines, and spends a considerable amount of time before the story starts in places like the universe of Usagi Yojimbo.

Film

 * A pretty obvious example would be The Adventures of Buckaroo Banzai Across the Eighth Dimension; crossing dimensions is easy with an Overthruster and a Ford F250.
 * And a rock band to back you up.
 * Ultraviolet has this as a Hand Wave for Violet's Hammerspace weapons.
 * From Beyond has another dimension that's a scary place.

Literature

 * Narnia is another dimension in CS Lewis's Chronicles of Narnia series of books, with specific rules about time. Indeed, the sixth book, The Magician's Nephew provides a very good fantasy description of dimensional travel, likening the space between worlds to the rafters in a block of townhouses. The titular magician also makes it clear that Narnia, Charn, and similar worlds have no geographical relationship to our world at all.
 * Peter F. Hamilton's The Night's Dawn Trilogy incorporates a wide variety of these (scientifically dubbed "continuum's"). The most prominent is the "Beyond", where  souls end up after leaving the body. It's non-spatial, but it has time, so that the souls of the dead are aware of the passage of time but have nothing to do but leech on to each other's memories for the feeble semblance of life that they have.
 * The Dark Continuum is as close to an actual Hell as it gets. This is a dimension of near-absolute entropy, where the souls of whoever ends up there are compressed into a zero-Kelvin mass of writhing agony called the Melange. In case you are wondering, yes, they are also fully aware.
 * There are also various "pocket universes", not much bigger in volume than a planet, where the Possessed transport the worlds that they steal.
 * The Lives of Christopher Chant by Diana Wynne Jones uses a similar "place between" which is clearly written in reaction to The Magician's Nephew's quiet, sleepy Wood Between the Worlds; it's misty, muddy, slippery and somewhat dangerous terrain. There are definitely no guinea pigs.
 * A Wrinkle in Time features a trio of mysterious guardians who are able to transport the protagonists through space via the fifth dimension. According to them, they are able to tesser, or "wrinkle," by bending space around so that they're in another place in an instant. As one character states: "A straight line is not necessarily the shortest path between two points."
 * Most of Clive Barker's stories revolve around traveling to and from another dimension, whether through a rug, painting, etc.
 * Book of Amber - And how. After walking a sentient maze and gaining the ability to do so (which nearly all the major characters have done), someone from either Amber or Chaos can walk from world to world, essentially willing the transfer from one to another. The transfer is gradual, but can do literally anything, including taking the traveler to a world whose mythology predicts the arrival of a deity who looks exactly like him or her. It's mentioned that no one is quite sure whether these dimensions actually exist before an Amber or Chaos resident enters it, but there is currently a sort of two-ended multiverse with Amber at one end and Chaos at the other, with the hundreds or thousands of worlds in between being more similar the closer they are, to both Amber and Chaos, and each other. And the laws of nature don't always work the same from one to another—for example, gunpowder doesn't ignite in Amber. Oh, and all of them except Amber and Chaos are called Shadows, because it's believed that they are only inter-dimensional shadows of the two true worlds. And how, indeed.
 * In The Boy who Reversed Himself one could get to the fourth dimension by learning to step 'ana' or 'kata' (the extra directions added to make it 4D) and needed special glasses in order to see more than floating blobs, as our eyes weren't designed for the dimension. "Ana" and "kata" are the ancient Greek words for "up" and "down," respectively.
 * Kay Kenyon's The Entire and The Rose series involves humans discovering a manufactured universe called the Entire. The beings in charge apparently copied sentient species from Earth's universe (the titular Rose) so all the creatures of the Entire supposedly have counterparts elsewhere in our universe that humans just haven't found yet. And there's trouble actually getting to the Entire from the Rose because the beings in charge refuse to share that information. (Those beings themselves and some mysterious attackers called Paion coming from two other universes.)
 * HP Lovecraft liked this idea and inserted it into many stories, especially the Cthulhu Mythos. It was used not only to explain where the various Eldritch Abominations hid from the world, but also to explain some of the Alien Geometries of the various structures and beings he created.
 * Robert A. Heinlein's Number of the Beast uses this as a Hand Wave for traveling The Multiverse.
 * The Myth Adventures series by Robert Asprin has multiple dimensions between which the protagonists often travel. Also, almost all the protagonists originate in different dimensions (Skeeve from Klah, Aahz from Perv etc.)
 * Kenneth Bulmer wrote a series about the Contessa Perdita di Monttevarchi, an interdimensional tyrant, and the various people who opposed here
 * According to Word of God, the races of the Codex Alera all arrived in the lands of Carna from other dimensions.
 * In Wicked Lovely: Sorcha's high court, most halflings and sighted ones,  live in a world known only as Faerie. It is also said that the dark court once resided there, but not during the events of the main series.
 * Other Dimensions connected to the Discworld include Death's Domain, Fairyland, The Dungeon Dimensions and Roundworld.
 * It is hard to take other dimensions more seriously than in Flatland.
 * The Jakub Wedrowycz stories feature at least one Another Dimension - it's a Medieval European Fantasy realm with some comedic twists.
 * The Territories in The Talisman. Both worlds tend to mirror each other such that doing one thing in one place causes a similar effect in the other. The inhabitants are also mostly the same apart from population differences.
 * In Jack Vance's Lyonesse trilogy, there is a long section set in Tanjecterly. It's a strange place where trees are different colors, and the heroine is menaced by grotesque, slime-eating creatures called Progressive Eels.

Live-Action TV

 * The demons of Angel and Buffy the Vampire Slayer often hailed from some hell dimension or another; our heroes on Angel have visited at least three of them. Most of them have different rules on time. For example, on Buffy a demon continuously captured teens to use as slaves, working them until they're in their old age, then finally dumping them crazed and confused back into our world—all of which happened in a matter of a day or two, Earth-time. Also, Connor was sent to the worst dimension imaginable as a child, and came out a couple weeks later as a teenager.
 * There is a running joke about shrimp entirely based on this premise, which has been liberally and enthusiastically embraced by online fandom at large: In "Superstar", when explaining the concept of alternate dimensions, Anya says: "You could have, like, a world with no shrimp. Or with, you know, nothing but shrimp."
 * In "Triangle", after Olaf was banished she said that he could have been sent to "the world without shrimp."
 * In the Angel episode "Underneath", Illyria talks about moving between dimensions, she said that she went to "a world with nothing but shrimp" but "tired of it quickly."
 * The episode "The Wish" introduces an alternate continuity timeline caused by Anyanka, which was supposedly destroyed when her demonic power source was destroyed. But it gets confusing because this alternate timeline is actually also an alternate dimension, since the episode "Doppelgangland" has AU!Willow being pulled from that universe into the primary universe.
 * Fluidic space, the area inhabited by Species 8472 in Star Trek: Voyager. It's an alternate dimension, only accessed through portals established in the region itself.
 * Sliders is built on this trope, with the first season showing different Another Dimensions each episode.
 * Doctor Who used this trope liberally in its 2005–present series, introducing "Pete's world," an alternate dimension with zeppelins but no Time Lords (who only exist in one dimension due to their wibbly wobbly Time Lord-y-ness), where Rose was never born and her father, Pete Tyler, never died. The show visits this dimension 3 times so far, twice in the 2006 season and again at the end of the 2008 season.
 * Another use came in the 2011 episode "The Curse of the Black Spot" where.
 * Previously, Adric came from Another Dimension called E-Space, which is also where the Doctor left Romana and K9 Mark II.
 * Also the Third Doctor found himself in a dimension where UNIT was tools of a Fascist British State and time was running slightly faster (this enabled him to prevent a disaster in our world after seeing how it engulfed a previous one).
 * Kamen Rider Ryuki and its adaptation Kamen Rider Dragon Knight both dealt with another dimension on the other side of Earth's mirrors.
 * Kamen Rider Decade traveled to multiple other dimensions - the homeworlds of other known Kamen Riders.
 * Hindu cosmology contains several universes and planets, so this may be Older Than Dirt.

Game Books

 * Most of the adventures in the Lone Wolf series take place on the world of Magnamund in the plane of Ao. There are other planes of existence such as the Daziarn, a strange dimension divided into mini-dimensions that have almost nothing in common, and the Plane of Darkness, which is basically Hell and the hometurf of Naar the King of the Darkness.

Tabletop RPG

 * Dungeons & Dragons (most notably Forgotten Realms) features planar travel, based on moral alignments, classical elements and various other things. Of course, the world that most of the action takes place in is the "middle ground". Or so all the Clueless berks think 'till they reach the Cage.
 * Mordenkainen's Magnificent Mansion!
 * White Wolf's Old World of Darkness games featured another set of dimensions called the Umbra, which was based very strongly on human perception, to the point where a shaman and a scientist in the same part of the deep umbra would see it as a surreal swirling nexus of spirit energy populated with arcane ghosts and a section of interstellar space populated by aliens, respectively.
 * The New World of Darkness has its own sets of dimensions. There's the Shadow Realm, which is like the Umbra, only it's nearly exclusively animistic. Then there's the Underworld, which is home to dead knowledge and concepts and the place where ghosts go when they've finished their business but aren't ready to pass on to their final reward. Then there's the Abyss, which is pretty much anti-reality. Then there's the five Supernal Realms, dimensions of pure magic. Then there's the Astral Realms, which are where the collective unconscious is made flesh. Then there's Arcadia, which is not the Supernal Arcadia and is a constantly shifting chaotic wasteland that plays home to The Fair Folk. Then there's the Hedge, the predatory gateway dimension between Earth and Arcadia. And on top of all that, apparently there's Hell.
 * Genius: The Transgression has smaller Bardos—pocket worlds made of concepts disproved by science. The more prominent ones include an alien-inhabited Mars, the Hollow Earth (home to dinosaurs and cavemen and Nazis), and the Seattle of Tomorrow, which Lemuria tried to bring into this world with disastrous consequences.
 * In Atlas Games' Feng Shui, players can travel through time by means of "The Netherworld", an alternate dimension made up of gray tunnels which lead to portals which allow access to and from our world at fixed points in time and space. The Netherworld is home to refugees from alternate timelines that have been erased from reality, including four siblings who ruled the earth in an Alternate History.
 * The Astral Planes of Shadowrun.
 * The world of The Dark Eye consist of seven planes, generally imagined as concentric spheres. The first is unaccessible and "only" a core. The second is a place of raw elemental powers. The third is the one where all the mortal life happens. The fourth is where the souls of the dead rest. The fifth is where the gods dwell, and cosists the paradises particularly worthy mortal souls may be eccepted into. The sixth is the sky with its stars, and some lesser gods can be found here. The seventh sphere is hell, a realm of chaos and infernal cold (ice/cold being the opposite of life in this world's elemental philosophy). In effect, the entire world, from the gods down, is just a relatively insignificant speck in a universe that wants to destroy it.
 * In Nomine is a natural for this trope, possessing not only a Heaven and Hell, but also a Dream Land known as the Marches, and even a "no-place" called Limbo for the souls of angels/demons who were killed and unable to return to their proper realm.

War Games

 * Warhammer 40,000's Warp is Another Dimension... 40k style. In essence, Hell. They use it for FTL travel. It doesn't always work. The ship might disappear then reappear, with everyone inside turned to dust from age. Or it might reappear hundreds to thousands of years later. Or appear at its destination before it left. "Time" is a funny thing in the Warp. Not funny Ha-Ha.

Video Games

 * Many Silent Hill fans agree that the games take place in a place which is like reality but in some crucial ways different, and the term 'alternate dimension' is a convenient term to describe this, though there are many interpretations of just what that actually means and whether 'dimension' should be replaced with some other, more accurate, term.
 * The Combine from Half-Life 2 are a cabal of dimension-spanning Planet Looters, and the "nearby"—in 11-dimensional superstring terms, at least—Xen border-world is the neighboring dimension by which we discover on our own. Xen itself is nebula-like with giant floating asteroids above a bottomless void (and copious amounts of Scenery Porn). At the end of Half-Life 2 we also get a glimpse of the Combine Overworld which looks like a hellish realm dotted with multiple Citadels.
 * Xen is also used a a strong plot point since it's a necessary component for Earth-made teleporters. As Mossman explains in HL2, the Resistance "figured out how to use Xen as an unexpressed axis, effectively a "dimensional slingshot" so that we can swing around the border-world and come back into local space without having to pass through". At the end of Half-Life, the dimensional breach left by the resonance cascade was relatively tiny but enough for the Combine who forcibly tore it open and invaded (with the side effects being destructive portal storms and copious amounts of Xen fauna). It was still open in Episode Two when the Combine tried to call in reinforcements but the rebels screwed up their plans and used Black Mesa's old satellite array to seal it permanently.
 * It's worth noting that Xen isn't a "proper" universe; it's described as a "dimensional travel bottleneck", and is so small, in fact, that it's atmosphere is dense enough to be breathable. Add to that the various chunks of planet and the xenofauna from a hundred different worlds, and the impression is that of a "bubble" of spacetime that someone happened to inflate and fill up with just enough material to allow habitability. It's not as unlikely as it sounds, given that we know the Nihilanth fled there to escape The Combine.
 * The Darkness features a hell like world that the Darkness resides in.
 * The new Wolfenstein game involves a Dark World-like dimension called "The Veil". It is our world, just viewed from an inch or so down the fourth spatial axis. But that's not the Axis you should be worried about.
 * There's also the Black Sun Dimension, a small, unstable universe being held together by the Black Sun at its center.
 * Galaxy Angel begins in EDEN, which consists of a lost civilization and the Transbaal Empire; Galaxy Angel II brings in two more dimensions, ABSOLUTE and NEUE.
 * The Nether in Minecraft. It is actually a quicker way to travel across the land. 1 Block traveled in the Nether is equal to 8 blocks traveled outside of the Nether. Sadly another gate must be made while in the nether as it is the only way for it to work.
 * The Myst games, which visit 'Ages' such as Stoneship (inhabited ship, embedded in an island), Mechanical (a clockwork fortress on the surface of the ocean), Riven (water on the five islands shies away from heat sources), Spire (flying, wind-carved ruinous mountains floating above a star), and Ahnonay (cleverly designed to appear to travel through time, to the uninitiated).
 * This may actually be a subversion. The prequel novelizations explain that if the universe is infinite, and planets are distributed relatively uniformly, all possible worlds must exist. The link books only transport the user to existing worlds which match the written descriptions, rather than creating new ones from scratch. This does not explain how Atrus is able to revise the Riven book continuously to delay the world's destruction.
 * One novelization indicates that there are two major philosophical views - that the dimensions are created by writing them, or that they're merely accessed. Characters who are on different sides of the dispute often come into conflict.
 * Two of the alien races from Star Control II come from Another Dimension: the Arilou come from Quasi Space, while the Orz come from a dimension that they refer to only as *below* (thanks to the trouble the Translator Microbes have with their language). The Arilou and the Orz might come from the same dimension, as the Orz say both of the two races are from *outside* and the Arilou are from *above*. It depends on the meaning of *outside*, though.
 * April Ryan of The Longest Journey jumps between "our" world (Stark) and the mystic Arcadia repeatedly throughout the game.
 * In City of Heroes, quite a few of the high-level missions involve visiting other dimensions or fighting invaders from them.
 * As well as the Shadow Shard, a series of 4 zones set in an alternate dimension that may very well be the mind of a god, inhabited by aspects of his personality.
 * Also, 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.
 * There's a very literal example in Super Paper Mario where Mario's special move is to "warp" the otherwise flat world, revealing its third dimension.
 * Don't forget Bestovius, Dimentio and Merloo, who all have dimension-flipping powers! It's quite popular in this game.
 * Two dimensions was featured in Super Mario RPG, 'The Factory' where the Smithy Gang game from, and 'Vanda' where Final Fantasy-inspired boss Culex originate. Only the former was visited, the latter was 'in-between'.
 * In the original SNES Star Fox, there is a secret level titled 'Out Of This Dimension' that has to be seen to be believed.
 * In Runescape, there exists a series of gates to the Fairy dimension Zanaris, which itself has a central 'hub' to travel to other, decidedly more hostile dimensions, such as the Abyssal Zone, Dimension X, which is host to horned kangaroos, and even a forest. A forest dimension.
 * Clive Barker's Undying has both Oneiros and Eternal Autumn, magical realms that are either enslaved or created by two mages in the story.
 * In the Sonic the Hedgehog series, while the two Sonic Rush Series games feature an Alternate Universe, Sonic Rivals 2 is about Eggman Nega plotting to free a demon that was trapped in Another Dimension
 * Some theories and other media sources of the Special Zone is Another Dimension. There's also the Twilight Cage from Sonic Chronicles, which seems to collect powerful civilizations from several dimensions.
 * Magicians Quest Mysterious Times has the spirit world, which crosses over with the real world during Mystery Time. During Mystery Time, new bugs and fish appear (including VAMPIRE SQUID), characters from the spirit world appear in the town and require your help, and Mr. Graves (the sleeping skeleton in the room with the organ and locker) wakes up to teach extracurricular classes. Oh yeah, did I mention that the sky turns an otherworldly shade of red?
 * Dark Aether of Metroid Prime is a Death Dimension that literally sucks the life from anything that enters it. It is home to the Ing, spiderlike Legions of Hell that possess creatures of the "Light World" so that they can enter it, as our dimension is just as lethal to them.
 * Used in the SNES Shin Megami Tensei games a lot. Majin Tensei II had 2 alternate dimensions.
 * Super Robot Wars NEO treats Earth Tear from Lord of Lords Ryu Knight this way.
 * The Super Mario World ROM Hack series The Second Reality Project features the titular Second Reality. The remake of the first game introduces Thirdspace into the plot.
 * DOOM
 * EarthBound has both Magicant and Moonside.
 * Kirby's Return to Dream Land has one that serves as an Amazing Technicolor Battlefield for the Big Bad of the game. It even uses the Trope Name.

Web Comics

 * This is the main plot of the comic Emergency Exit.
 * Sluggy Freelance introduced the Dimensional Flux Agitator, a device that opens portals to other dimensions at random, in its second chapter. The device has been brought back many times since then, to the point where a full fledged multiverse has developed.
 * El Goonish Shive has a sub-plot involving alternate dimensions - however it's really just used as another name for Alternate Universes.
 * Unicorn Jelly and its spinoffs are set in other-dimensional realms with their own unique physics.
 * In Planes of Eldlor, demons from another dimension are trying to break into the world.
 * This is the premise of Between Two Worlds.
 * Richard once got banished to the Plane of Suck.
 * As of Act 6, Homestuck has a grant total of  parallel universes, with another mentioned in backstory.   Each universe also has an Incipisphere attached to it. The Incipispheres are separate from each other, but all of them are in the Furthest Ring outside normal reality. And there's also "the real world", which is inhabited by the author.

Web Original

 * The Gamers Alliance has several planes of reality, including the Land of the Living, the Land of the Dead, the Void, the High Plane, and the Demon Realm among others.
 * Creturia, the parallel world from Dimension Heroes.
 * Similar to Sliders, the whole point of AH Dot Com the Series.
 * Adylheim has a great deal of these, including a cosmarchy spanning five alternate realities.
 * Felarya is described as a "dimensional plane". Hell and Heaven are both separate dimensions, which mimic the various belief systems of those who arrive there.

Western Animation

 * Teenage Mutant Ninja Turtles had a villain who came from "Dimension X", a world accessible through glowing portals.
 * The anniversary movie Turtles Forever posits that every incarnation of the Teenage Mutant Ninja Turtles comes from a different dimension, and that there is a "Turtle Prime" dimension from whence all TMNT dimensions originate.
 * One famous Halloween episode of The Simpsons has Homer entering the third dimension. After causing that dimension to collapse, he ends up in the "real" world, "the worst place yet."
 * Like many Simpsons Halloween sketches, this was a parody of a Twilight Zone episode.
 * Raven of the Teen Titans comes from one of these. She has threatened to deport Beast Boy to Another Dimension. She goes to what certainly looks like Another Dimension to deliver a glorified cell phone. Maybe it's so easy for her because
 * In the Real Adventures of Jonny Quest episode "Other Space," scientists open a portal to another dimension, complete with inhabitants that want to take over our dimension.
 * In the WITCH TV show and comics that inspired it the Five Girl Band are appointed as Guardians of the Veil (later  to police their Multiverse and prevent various bad guys and the occasional Eldritch Abomination from messing things up. In this case "dimensions" seems to mean planes of existence instead of alternate universes as Human Aliens are rarely encountered. Most dimensions fit the fantasy archetype to some degree or another but one world in the comics was an almost literal case of Alien Geometries.
 * The Cowboy Universe from Futurama, which is identical to our own, except everyone is dressed like a cowboy. While it's described as the only parallel universe in existence, later episodes RetConned it. So it's most likely an Alternate Dimension, given that it's viewable from the edge of the universe.
 * Word of God says that the cowboy universe is the only PARALLEL universe. The others were PERPENDICULAR universes.
 * Avatar: The Last Airbender has the Spirit World, a dimension parallel to the one the characters live in. It's home to most of the Avatar world's spirits and seems to operate on a completely different kind of physics. The titular Avatar is the bridge between the show's two dimensions, as a spirit who takes on human forms to better understand humanity. We only ever see Aang visit the Spirit World, but it's heavily implied that Iroh has been there as well, presumably indicating anyone can get there if they try hard enough. There are also several spirits (Wan Shi Tong, Twi, La, and Hei Bai) who have visited the physical world.
 * Some fans theorize that when people die, they spend time in the Spirit World before reincarnating.

Real Life

 * In string theory, which is the highly speculative Hot New Thing in theoretical physics for the last few years, our visible cosmos is located in a ten- or eleven-dimensional hyperspace, which may contain an arbitrary number of other continua, with varying kinds of matter, forces, and numbers of dimensions.