The Wall Around the World

All around where you grew up is a barrier. No one knows what lies on the other side. Or if they do, they're not telling. It could be Here There Be Dragons, or your ancient enemies, or it could be that you and everyone you know is Sealed Good in a Can (or evil, who knows?). Passage through will be difficult if not impossible, for what good is a barrier if anyone can walk on through?

When neighbors from another *continent* get too noisy.

The wall can surround a single village, a town, a continent, a world, or even an entire galaxy. Or it could seemingly surround nothing, and simply mark a barrier between one world and the next. Surprisingly common in Soviet era SF. Think about it.

Note that, despite the name, the barrier does not have to be a literal wall.

If the barrier surrounds a community, it is an isolated Small Secluded World or City in a Bottle or possibly a Domed Hometown.

Examples of The Wall Around the World include:

Anime & Manga

The wall around the town in Haibane Renmei. (We never do find out what lies beyond, though considering that the walls are death...)
In the first and second seasons of Slayers, the world Lina could explore (and put craters into) was restricted by a magical barrier that went down after the Big Bad powering it was killed.
The wall in Princess Tutu is both literal and metaphorical, keeping reality from intervening in the narrative-controlled Gold Crown Town. Most people don't even realize it exists, since the story prevents them from wanting to leave. (This doesn't stop people from suddenly appearing inside the town gates, but it's ambiguous whether they're capable of leaving.)
Tengen Toppa Gurren Lagann has the human villages deep underground. The planet's surface is overrun with monsters, and humanity has hidden away for so long that most of the people in Kamina's village question whether the surface actually exists.
In the oneshot manga Island, by Komi Naoshi, the town the main characters live in is surrounded by a huge wall, much like a well. When the islanders turn 14, they are shown the truth- outside their island is nothing but a vast sea. The islanders believe that all the land in the world sunk and thus all other countries were drowned, making it useless to go outside the island. It turns out that only the island sank, probably because of land subsidence and earthquakes.
Tokyo Jupiter in RahXephon, encasing Tokyo (and looking like Jupiter).
A variety occurs in Angel Beats!. There's no literal wall, but the world around the high school complex just disappears into a thick fog once you travel beyond the hills.

Comic Books

The Incredible Hulk occasionally visited the Keystone Quadrant in his old comic-book series... basically a solar-system (possibly more than one) which was somehow 'walled off' from the rest of the universe, it could only be entered and exited through various types of teleportation. It was basically a Sugar Bowl without the sugar - populated by funny talking animals and hilariously incompetent Keystone Kops... and caught up in a long war between a Mad Scientist tortoise and his cybernetically-enhanced Black Bunny Brigade (not to mention a small army of robotic Monster Clowns), and the heroic Animal Resistance, led by a fast-talking Raccoon space-captain.
The Source Wall is a wall around the entire DCU, which...well, who fucking knows. It makes no sense. Either 2D Space is in full effect or it lines the entire interior of the universe, in which case the universe it both finite and shaped in a way where that makes sense. Also, there are powerful cosmic beings embedded in the wall, and The Source (which may or may not be God) is on the other side.

Film

The forest containing The Village is closed off from the outside world by a wall. Turns out there's a reason for that.
The desert that surrounds the Maitlands's house in Beetlejuice.
The walls of Truman's enclosed world in The Truman Show.
The broken bridge in Dellamorte Dellamore, aka Cemetary Man.
In Dark City, John Murdoch tries to reach Shell Beach; instead he finds a wall at the edge of the city.
In the film version of Aeon Flux the survivors of the "industrial virus" (biological apocalypse) have lived in the walled city of Brenga for generations. The outer perimeter of the wall is periodically sprayed with some sort of poison to keep the outside world at bay.
A (probably apocryphal) story about Harlan Ellison's pitch for the first Star Trek film claims that Ellison met with Paramount executives and provided an outline for an epic story which ended with the crew of the Enterprise traveling to the edge of the universe, encountering a massive wall there, blasting a hole through it with their phasers, and seeing the eye of God staring back at them. Studio heads, however, were unimpressed, claiming that the premise wasn't "big enough", at which point Ellison stormed out of the meeting.
In The Last Starfighter, the entire civilized-good-guys portion of the galaxy is surrounded by an enormous force field called the Frontier. The evil Ko-Dan Armada lies outside the Frontier, but they've found a way to drill through it. (Cue Musical Sting.)
In The Thirteenth Floor the world has no physical wall around it but it does have an edge where the simulated nature is visible to the naked eye. People with in the simulated world are just programmed to never think about going anywhere near that edge (of course there are exceptions...)

Literature

The Trope Namer is a short story by Theodore R. Cogswell in which it separated a magic-dominated half of the world with a science-dominated one.
The one located in the town of Wall in Neil Gaiman's Stardust.
In The Sword of Truth / Legend of the Seeker, there is a (almost impenetrable) great barrier around a region called "The Midlands", which is the central geography of the story.
- That barrier is also re-used in Naked Empire of the same series, to close off a group of people from the rest of the world.
There's one of these in Garth Nix's Old Kingdom books, separating the nonmagical land of Ancelstierre from the Old Kingdom, where there's necromancy and other magic. It's actually an artefact containing one of the five Cosmic Keystones that keeps the Charter together and is designed to keep anything nasty inside the Old Kingdom where people know how to deal with it. It's only moderately successful, hence the massive trench and bunker network on the Ancelstierran side.
Ian McDonald's Out on Blue Six—the city is surrounded by a giant Wall, and the protagonists explore to see what's on the other side. Turns out--nothing but toxic waste.
In Damon Knight's Hell's Pavement, people in Connecticut (200 years in the future) know nothing of the people in New York, who know nothing of the people in Ohio, and so on. They believe people in the other places are literally monstrous and inhuman. (There are walls between zones.) This happened because supermarket chains used brilliant new brainwashing techniques to make people totally loyal to their brands, and the adherents of different brands formed different zones.
The planet Krikkit in Hitch Hikers Guide to The Galaxy was surrounded by a thick fog such that they never saw outside their world. This was done by the remnants of the supercomputer Hactar, making the Krikkiters into an Omnicidal Maniac race once they saw the universe. He did this so they would use the universe-destroying bomb he had invented, thus fulfilling a duty he welched on long ago and geting rid of his long-standing guilt.
- In So Long and Thanks for All the Fish, Wonko the Sane builds an inside-out house he calls "the Asylum" to fence in the rest of the world (he, naturally, lives "outside the Asylum", which is inside the house). He'd decided the entire world had gone insane when he came upon a pack of toothpicks with instructions.
In the novel Hard-Boiled Wonderland and the End of the World, the End of the World sections take place in a town which has a wall around it, and once you come to the town you can't go outside the wall.
If you only follow the first book, Oz would seem rather like this. The endless deadly desert surrounds Oz on all sides, isolating it rather nicely. Too bad later books place other magical kingdoms on the other side of a desert that seems rather more like a moat. Eventually, all the magic-users in Oz gather their power to put a wall of invisibility, thus more permanently sealing off Oz.
There's a short story by Arthur C. Clarke called "The Wall of Darkness" about a planet with a wall that divides it in half. The protagonist attempts to climb the wall too what's on the other side. turns out there is no other side, and the planet is essentially a 3D moebius strip, and so only has one side
The Void in Peter F. Hamilton's Void Trilogy, arguably.
A global glacier surrounds the only habitable continent on all of Darkover, literally called The Wall Around the World by the inhabitants.
In The Singer of All Songs, the order of priestesses known as the Daughters of Taris live surrounded by a giant wall of ice. They are the only people who can use ice magic, so they control who can come in and out.
The great Agatean Wall in Interesting Times in more to keep everyone inside, rather than other people out. According to the leaders, there is nothing but ghost and vampire filled wasteland outside it.
In the Dosadi Experiment the whole eponymous planet is encased inside "God Wall" barrier as a part of said experiment. Not that it's completely impassable, but for most people inside it is.
The Land of Elyon, a children's series by Patrick Carman, has walls surrounding the inhabited cities and the roads that link them. The main character finds a way out of the walls, despite the fear of many of the other characters about what is beyond the walls.
The Green Wall in Yevgeni Zamyatin's We, separating the civilization of the One State from the forests around it, which in turn separate them from the rest of the world. We are given few and conflicting clues as to what actually may exist beyond the forest.
Ted Chiang's "Tower of Babylon" is a speculative fiction short story where it's more of a ceiling or floor. The vault of heaven is a literal stone roof to the universe, and the Babylonians have built a tower to talk to God, who they believe resides above it. One of them makes it, only to emerge from a cavern deep in the Earth, back where he started--somewhat similar to the Clarke example above, the world loops back on itself.
Marlen Haushofer's "The Wall" is about a woman one day waking up in a mountain valley with the whole valley suddenly surrounded by an invisible, impenetrable wall. With all life outside the wall apparently dead, the book deals with her trying to survive inside the valley. Wondering if she is the last human alive, she speculates about the origin of the wall, which in the end is never revealed. She often thinks about trying to leave the valley, but never can't bring herself to risk it. What happens with her in the end is left open to the reader.
Perry Rhodan uses this on a number of occasions (including a 'wall' around the entire Milky Way Galaxy that the protagonists had to deal with after losing a few hundred years in an unexpected stasis field while outside, once). There's also a more literal example in Wardall, a tide-locked planet with a wall running around its entire circumference following the terminator. The planet's former natives apparently lived inside said wall rather than on either side of it, not surprising considering the conditions there; by the time the issue set on the world opens, though, its only inhabitants are the surviving crew members of a crashed pirate vessel and their descendants.
The wall separating Experiment House property from Narnia in The Silver Chair.
A literal example is the spherical Walls of the World from JRR Tolkien's legendarium, which are only specifically described in The History of Middle Earth although their existence is implied in The Silmarillion. The walls separate the world from the empty void of the Outer Dark, and are only pierced by a single Door of Night, created by the Valar to thrust Morgoth out until The End of the World as We Know It.
A large portion of the plot in Orson Scott Card's Pathfinder revolves around one of these. It's revealed decently early on that there are actually 19 "worlds" with Walls.
The world of Ethshar is a Flat World, being the end-cap of a cylinder. The edge of the end-cap is marked by a "noxious yellow gas".

Live Action TV

The barrier around the galaxy in the Star Trek: The Original Series episode "Where No Man Has Gone Before".
- Another old series show, "For the World Is Hollow and I Have Touched the Sky", where the world is a hollow asteroid.
In the Doctor Who story "Inferno", the Doctor pushes through a barrier in time and ends up in a Mirror Universe.
- Also used in the DWM comics, most notably in "Oblivion".

Myth, Religion, and Legend

Jericho, from The Bible, is now synonymous with its absurdly strong fortifications. Tends to happen when it takes God Himself to bring them down.
- There is also reference to a the sky being a firmament, a literal wall around the entire world.
  - Prior to the Flood. It pretty much fell down then.
A pre-Islamic Turkic myth has the Turkic people fleeing into a valley surrounded by mountains of iron to survive an onslaught. Their point of entry collapses, effectively sealing them from their enemies and letting them stay there for generations. When they decide to leave, they do so by melting the iron mountain.

Tabletop Games

The borders between the physical realm and the spirit worlds in the Old World of Darkness RPG line (the Gauntlet and the Shroud) qualify. Most humans have no idea that the spirit realms are real.
- The Gauntlet still stands in the New World of Darkness, cutting off the Shadow from the material. There's also the Abyss, which severs the Supernal from the Fallen.
Dungeons & Dragons, as always:
- The Misty Border in the Ravenloft setting cuts it off from the rest of the multiverse. You can check in, but you never check out. Darklords can do this at will (with few thematically-appropriate exceptions) to isolate their own domains.
  - The town of Barovia has its own permanent version of its domain's closed border; only the Vistani know how to make a secret antidote that allows safe passage.
- Spelljammer has a borderline case: crystal shells. Oh, it can have many thousands of portals... spread over the whole surface of a star system, that is. It's not easy to find one without knowing where it is, and they don't always stand still forever. Thus the proper magic is the best way to locate a portal or even open temporary one—for those who have it.
- However, Athas, the world of the Dark Sun setting, seems cut off from the rest of the Dungeons & Dragons universe. The most the Spelljammer setting would say about it was that "Athasspace" was "not on the spacelanes", but hints that there might be some way to get there. Planescape mentions it a few times, hinting it's possible to go there via portals in Sigil. It's likely TSR wanted to discourage players from traveling from, say, Oerth to a low-magic world where slavery is not considered evil and both iron and water (two materials considered priceless on Athas) are common; such could be a disaster waiting to happen.
The Weirding Wall in Nobilis which encloses the whole universe.
Paranoia is set in Alpha Complex, a domed city. The existence of "Outdoors Sector" is acknowledged, but information about it is limited, especially at low security clearances.

Video Games

Custom Robo (the Gamecube version) has the humans live inside a domed city that isolates them from the post-apocalyptic world. The outside world is kept secret except to a select few. But when you beat Rahu III, the final boss, it is revealed to everyone.
In Grandia, an entire continent was divided by an enormous wall about a mile high. No one ever tried to explain why.
- Could have something to do with Gaia killing almost everything in it's path, as it's only encountered on that side of the wall until it got on an airship.
- Grandia II had something similar, a huge nigh-uncrossable canyon, though its existence was explained: it was basically caused by God crashing into the earth.
In Wild ARMs 4, your first indication that Ciel is not a typical RPG hamlet is when fighter craft shatter the barrier surrounding it that was disguised as sky. The outside world is quite a bit different.
City of Heroes has the War Walls, justified as barrier against alien invasion, but really there as a level separation.
Palm Brinks in Dark Cloud 2 was sealed off from the rest of the world via a titanic wall, far too tall to scale. This was done by the Mayor, to protect the citizens from the incredible devastation taking place in the outside world—but now that the land is healing (and with the heroes having escaped via an underground sewer/aqueduct,) many of Palm Brink's inhabitants dream of exploring and building new cities.
Star Control 2 has slave shields—barriers around homeworlds of defeated races who don't want to fight on Ur-Quan side.
The Legend of Zelda: Wind Waker: The Great Sea has no physical barrier to keep you from leaving the map. However, your boat tells you that it's dangerous to leave and turns you around.
- The Legend of Zelda: Skyward Sword has an impenetrable cloud cover that separates the residents of Skyloft from the surface. As far as the people of Skyloft are concerned, the "surface" is a mythical place, rumored to be filled with monsters.
Gensokyo, the setting of the Touhou games, is walled off from the Outside World by the Great Hakurei Barrier to preserve Youkai, though people and objects occasionally slip through (particularly things the outside world has stopped believing in).
There's no actual wall on Hillys in Beyond Good and Evil, but if the player strays too close to the edge of the map, a series of pillars will rise up out of the water and warn the player that they're leaving territorial waters. Trying to get past them will just lead to them shooting non-lethal lasers at the player's vehicle to turn it around.

Web Comics

The area known as The States in White Noise is surrounded by a gigantic wall and poison gas filled moat. No one is allowed in or out except for bounty hunters, and residents hate and fear those who live beyond it.
In Sluggy Freelance the "Punyverse" turned out to be surrounded by a giant solid sphere, the inhabitants mostly didn't know that and thought it was an endless void inhabited by "void ghosts" that occasionally attacked (it was really wild shots reflecting off the sphere). Also their entire universe was artificial

Western Animation

The Simpsons split the school into the clean, cutesy "girls" side and the rough, rowdy "boys" side.
- Also, the glass dome enclosing Springfield in The Movie.
- And the wall made of garbage separating Springfield from New Springfield.
In Futurama, the Planet Express crew visits the Edge of the Universe, which has a convenient viewing platform. They are able to look through binoculars at the Universe Next Door, (which is apparently cowboy-themed).

Fry: Wow. So there's an infinite number of parallel universes?
Professor Farnsworth: No, just the two.

In South Park episode "Pinewood Derby", Earth and the Moon are sealed off by a cube-shaped force field after the humans fail the Space Cash Test.
The Duckman episode Exile in Guyville had a wall being built down the middle of America, dividing the sexes with Women on the East and Men on the West.
Ba Sing Se, the Earth Kingdom capital in Avatar: The Last Airbender, is surrounded by two giant walls. People within the city are generally encouraged not to even think about the world outside the walls.