Universe of Kingdom Hearts
In the Universe of Kingdom Hearts, also called the World, there are many worlds that fall into certain realms. All of these realms are on their own plane of existence.
Legend has it that long ago, there was one single world that was constantly bathed in the warmth of the light of Kingdom Hearts. In the "age of fairy tales", the world was whole and full of light, which was believed to come from Kingdom Hearts, which was protected by its counterpart, the χ-blade. Over time, people came to desire the power of Kingdom Hearts, introducing darkness into the World.
In order to take control of Kingdom Hearts, many people forged Keyblades in the image of the χ-blade, and they clashed together in a far-reaching conflict that embroiled the entire World. This event became known as the Keyblade War. In the end, darkness covered the World, the χ-blade was shattered into twenty pieces, seven of light and thirteen of darkness, and the "true" Kingdom Hearts disappeared into the darkness.
However, the world was restored by the light within the hearts of children, as many individual worlds separated from each other. The remnants of the battlefield on which the Keyblade War took place became the Keyblade Graveyard, a scarred wasteland filled with the Keyblades of fallen warriors. The remaining Keyblade wielders resolved to protect the World from further destruction, becoming guardians of the world order.
Each world that was recreated following the Keyblade War has its own time flow,  but as a result, their rebooting is left at random paces. During the reboot, Heartless run rampant and have to be slain by Keyblade wielder to prepare the world for the inhabitants' return.
Prior to the release of the Heartless, the worlds were surrounded by barriers to prevent outside interference, making it impossible for different worlds to have contact. However, the arrival of the Heartless caused the barriers between the worlds to crumble and disappear, and some worlds were consumed by the eldritch horde. Gummi Blocks are pieces of these broken barriers.
Those who travel between worlds are advised to avoid meddling in the affairs of other worlds and informing their inhabitants of the existence of other worlds. In order to maintain a balance of separation, several characters, notably Sora, Donald, and Goofy, magically alter their physical appearances in certain worlds to avoid standing out. For example, they take the forms of sea creatures in Atlantica, monsters in Halloween Town, jungle animals in Pride Lands, and computer A.I.s in Space Paranoids and The Grid.
The Ocean Between, also known as the "ocean of stars/interspace" (星の大海, Hoshi no Taikai?) and "sea between worlds/Other Sky" (異空の海, Ikū no Umi?, lit. "Sea of Other Skies"), is a vast space between the worlds. It is threaded by the Gummi routes and lanes between, which serve as paths between worlds for those with the knowledge to use them.
Traveling between worlds is normally very difficult. In addition to being separated by the Ocean Between, worlds are also protected by barriers which prevent outside interference. When Heartless attack a world these barriers are broken, indicated by a meteor shower in that world. The barriers are restored when the worlds are restored and their keyholes locked. The loss of the barriers allows virtually anyone access the world without assistance. With the barriers in place, only certain people, such as Keyblade wielders and those with access to Lanes between or Corridors of Darkness, can gain access to worlds.
In Kingdom Hearts, Kingdom Hearts II, and Kingdom Hearts III, Sora, Donald, and Goofy use the Gummi Ship to travel between worlds on the World Map, an event that serves as a mini-game between areas. Some worlds require certain special Gummi Blocks to access; for example, a special Navigation Gummi must be installed by Cid to unlock a second set of worlds in Kingdom Hearts. Once they reach them, the party can enter and then depart each world through its Save Points.
Certain worlds cannot be accessed by Gummi ship as they are located within other worlds or on other planes of existence. Examples of these are the Data worlds, such as Jiminy's Datascape and Space Paranoids, 100 Acre Wood, Dive to the Heart and the Final World. In Kingdom Hearts Chain of Memories, the worlds Sora visits are recreations from his memories from the first game brought to life on each floor of Castle Oblivion.
The Lanes between (異空の回廊, Ikū no Kairō?, lit. "Corridors of the Skies") are a series of "roads" between the worlds in Kingdom Hearts Birth by Sleep that were unlocked by Eraqus. Ventus, Aqua, and Terra frequently use these paths to navigate the worlds. The presence of Darkness is very strong within them, forcing travelers to wear their Keyblade Armor as a safety precaution. To enter the Lanes, the Keyblade wielder raises his or her Keyblade to the sky, and creates a portal leading into the Lanes, appearing somewhat similar to the Dark Corridors. Upon opening the way, they usually call their "Keyblade Glider" and use it to enter. The Lanes resemble the Corridors of Darkness, but with a color scheme closely resembling the Aurora Borealis.
Some very powerful Keyblade wielders, such as the Foretellers, have the ability to create portals to the lanes between without the use of a Keyblade.
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Corridors of Darkness
The Corridors of Darkness (闇の回廊, Yami no Kairō?) are pathways that interlink the various worlds, as well as connecting the Realm of Darkness and Realm Between to the Realm of Light. They may only be opened by those who have given themselves over to darkness, but can be used by almost anyone once opened. The Heartless and Nobodies are attempting to link their own Corridors to the gates. Corridors of Darkness also occasionally form spontaneously within the Realm Between, often when a world is destroyed and its surviving inhabitants are cast away to worlds like Traverse Town.
While nearly anyone can use the Corridors, frequent use will erode one's body, so those who rely on the Corridors, such as Organization XIII, wear black coats to protect themselves. Those without the coats can survive occasional exposure to the Corridors' darkness if their hearts are strong enough.
Gummi routes and Gates
Pieces of Gummi known as "Star Shards", or the Gummi Ships built from them, are able to travel the Ocean Between. Using Gummi Ships is much easier, as single Star Shards will cause random jumps if their handler isn't skilled in their use. Although the closely-guarded lanes between already allowed some limited travel between worlds, the gummi routes were made ubiquitous when the Heartless first began destroying the walls between worlds, leaving enormous holes that made it possible to travel for anyone who could get their hands on Gummi. The holes also made it possible for anyone to travel without the assistance of Keyblade wielders.
However, when the walls between the worlds were restored with the destruction of End of the World, Gummi Ships became all but useless. In response, the worlds themselves prepared the Gates (ゲート, Gēto?) as new pathways, or "highways", for Sora and his friends to travel. The Gates allow their users to travel between worlds despite the presence of the otherwise impassable walls between them, making them similar to the lanes between. The first Gate known to be used, leading to the Land of Departure, was opened by the Wayward Wind after Aqua rescued Ventus's comatose body. As Yen Sid informs him, Sora is able to unlock them when his Keyblade reacts to a specific object significant to its world, opening a new Gummi route. Though after Kingdom Hearts II, these new paths sealed themselves. 
After Sora's failed Mark of Mastery exam, he travels to Olympus alongside Donald and Goofy by opening a Gate simply by using his Keyblade, without an item triggering it. Once Master Xehanort obtains the χ-blade, he is able to open Gates without items triggering them, which he uses to separate the Guardians of Light from each other. Soon after, Sora opens a Gate to locate Kairi, by using the crystallized pieces of her heart that Xehanort had held captive in Scala ad Caelum as his trigger.
Certain sorcerers, such as Merlin, are able to instantaneously teleport themselves between worlds, apparently without using other methods. Merlin and Yen Sid are also able to open passages to past versions of a world, allowing travelers to move through both space and time.
With his magic, Merlin is able to bypass the normal rules for time travel. Anyone else attempting to do so like Xehanort have to cast off their bodies and travel in one direction through time as a heart. Yen Sid shows greater power, having sent Sora and Riku back to the beginning of Kingdom Hearts for their Mark of Mastery exam and reverting their ages by a year.
In the Kingdom Hearts games, each Star is a specific world. Should a star stop shining, that would mean that a particular world has disappeared from existence, most likely from destruction by the Heartless.
In Kingdom Hearts, many stars disappeared. In Mickey's first letter to Donald, Mickey mentions the fact that the stars have been disappearing, one by one, and that he left to find the cause. When Donald and Goofy first arrived at Traverse Town, they discovered a star corresponding Destiny Islands fade out of existence. Some of them were restored after Sora and his friends stopped Ansem's plan to plunge the world into darkness, but others remained submerged in sleep as part of the Sleeping Worlds.
In Kingdom Hearts II, Mufasa implies that there are stars to represent certain individuals as well, such as when he told young Simba that all the great kings of the Pride Lands are looking at them in the form of stars. Years later, Simba tries to reach out to his deceased father, and he appears to him immediately. The same implication happens in Kingdom Hearts Birth by Sleep, when Master Yen Sid detects that "Eraqus's star has blinked out," after he was struck down by Master Xehanort.
Realm of Light
The Realm of Light (光の世界, Hikari no Sekai?) is the portion of the universe with an affinity for light and is positioned opposite the Realm of Darkness. Like the Realm of Darkness, the Realm of Light has a Keyblade, the Kingdom Key, which chose Sora as its wielder. The Realm of Light is the primary setting for the series, although the protagonists sometimes travel to the Realm of Darkness, the Realm Between that connects them, or the Sleeping Worlds that lie parallel to them all.
Door to Light
The door leading from the Realm of Darkness to the Realm of Light is called the Door to Light (光の扉, Hikari no Tobira?). This was the door Sora and Riku went through while they were stuck in the Realm of Darkness. It is not certain what caused the door to appear, though as Riku suggests, it could have been due to the light within Sora's heart which was brought out by Kairi's Letter. Only those from the Realm of Light may pass through the Door to Light.[?]
Realm of Darkness
The Realm of Darkness (闇の世界, Yami no Sekai?) is a realm that lies opposite the Realm of Light. There are few people who have lived or are living here, such as Aqua, Ansem the Wise, and Riku, and it is instead inhabited by the Heartless and other dark beings. The only known Keyblade of the Realm of Darkness is the Kingdom Key D, wielded by Mickey. The Kingdom Hearts of Worlds' Hearts began growing in the Realm of Darkness after the Heartless plague was unleashed on the Realm of Light.
Door to Darkness
The Door to Darkness (闇の扉, Yami no Tobira?) is the dark counterpart of the Door to Light. It is a tall, white door that lies within Final Door, at the End of the World created by the Heartless, and can only be opened by the Princesses of Heart. Beyond the door is the Realm of Darkness where their artificial Kingdom Hearts of Worlds' Hearts resides. Only those from the Realm of Darkness, without light in their hearts, may pass through the Door to Darkness.[?]
After the battle with the World of Chaos, Ansem, Seeker of Darkness opens the door and asks that Kingdom Hearts fill him with the power of darkness. To his horror, Kingdom Hearts is revealed to be light and it destroys him, but darkness begins spilling out. As Sora and his friends struggle to shut the door, they see Mickey and a restored Riku on the other side, and together, they manage to close the door. Just before they are separated, Mickey assures the others that they will find the Door to Light and be reunited.
During his summer vacation, the battle at the door reappears within Roxas's dreams as part of Sora's memories.
Later, as they prepare to escape Space Paranoids, Tron informs Sora that he needs a password to access the computers dataspace, the "DTD". Back at Ansem's Study, Sora finds a clue behind a portrait of Xehanort which indicates that "DTD" is short for "Door to Darkness". Just then, Mickey enters and speculates that this means the password to unlock the DTD should be the names of the Princesses of Heart. After using that password to access the DTD, Tron changes the password to the names of Sora, Donald, and Goofy in order to protect it from the MCP.
The Realm Between (狭間の世界, Hazama no Sekai?) is the third realm in the Kingdom Hearts series and lies on the boundary between the Realms of Light and Darkness. Few worlds exist within it, and most have some special nature or purpose.
As of now, Traverse Town, Land of Departure/Castle Oblivion, Twilight Town, Mysterious Tower, and The World That Never Was are the only worlds confirmed to exist within the Realm Between. Each world ties closer to the Realm of Light or the Realm of Darkness, with the exception of Twilight Town, which exists exactly in the middle.
Realm of Sleep
The realm of sleep, sometimes referred to as the realm of dreams, is a place that can usually only be entered when someone is asleep or unconscious, though it is sometimes possible to physically travel there under certain circumstances. It contains the worlds of dreams and memories, such as Dive to the Heart, which are directly influenced by the dreaming individual. Anything that happens in these worlds can also affect the waking world. The Final World, being on the edge of reality just before a heart ceases to exist, is nested closely to the realm of sleep. Those who dream have a chance to catch a glimpse of The Final World, as Sora does when he begins his journey.
The Sleeping Worlds (夢見る世界, Yumemiru Sekai?, lit. "Dreaming Worlds") are worlds submerged in sleep. They were created following the defeat of Ansem, Seeker of Darkness, as a portion of the worlds that were attacked by the Heartless remain submerged in sleep. Because this created barriers, not even the Heartless could penetrate, darkness took a new form in the Sleeping Worlds, known as the Dream Eaters. Sleeping Worlds can only be entered when someone dives into the realm of sleep.
Datascapes are realms made from data where data worlds exist. A data world may be created using data from a real world, thus creating a digital replica of that world, or may be programmed to be unique. Data worlds cannot be reached via Gummi ship as they usually exist within computers and can only be accessed via an interface within the world that contains them.
The Other Side
The other side, or what Ansem the Wise termed "Unreality" and hypothezied as a world of fiction, is a world that exists on the other side of reality. During her year long sleep, Kairi searched her memories in an attempt to find a clue to Sora's whereabouts. She ultimately found a memory of Terra-Xehanort sending her to Destiny Islands when she was a child, where he describes a world that is "neither of light nor darkness, but somewhere on the other side". This, along with Riku's dreams of Sora, lead them to the Final World, where the Fairy Godmother introduces them to the Nameless Star, who is an inhabitant of the other world. Riku is able to use the power of waking on the star to gain access to the world. The only known place in the other world is the city of Quadratum, the home of Yozora.
A world (世界, Sekai?) is a location that appears in the Kingdom Hearts series. Serving as the "levels" for the games, Sora and other characters are charged with visiting and protecting them through the course of the series. By defending the worlds from the Heartless, Nobodies, or other enemies, Sora keeps the darkness from spreading and the worlds from being manipulated or destroyed. At the end of every world, Sora, Donald, and Goofy have to fight a boss of that world (which is either a giant Heartless or a Disney villain), which opens the Keyhole area when defeated. At the end, Sora locks the Keyhole and earns a Gummi that will let him go to the next world.
Each world that appears in the Kingdom Hearts series has its own distinct look and is reminiscent of the films and cartoons they are based on. Oftentimes, the plot of the game in that world will be based on its original source. Almost all worlds can be visited in the course of the game and are part of the playthrough; there are exceptions, though, since some places can be seen in cinematics, but not actually accessed by Sora's party. All worlds are separate from one another, and with a few exceptions, most people are unaware of the existence of other worlds.
There are over thirty worlds that have appeared in the Kingdom Hearts series so far. Most of them are based on Disney's animated features and contain locations and characters from or inspired by the films. As a result, worlds are greatly varied in appearance, style, and nature, as their sources range from the oldest cartoons created by Disney to more current films in the Disney canon. The oldest source is the 1928 cartoon Steamboat Willie, used as a basis for Timeless River, while the most recent inspiration for a world from Disney's animated features is 2014's Big Hero 6. In Kingdom Hearts II, two live-action films, Pirates of the Caribbean: The Curse of the Black Pearl and Tron, served as the foundation for the worlds Port Royal and Space Paranoids, respectively, while Kingdom Hearts 3D: Dream Drop Distance included "The Grid" based on the live-action film TRON: Legacy.
In addition to the Disney-based worlds, twelve original settings were created by game developers Square-Enix as worlds unique to the Kingdom Hearts series. They tend to be populated by Square-Enix characters from the Final Fantasy series, as well as Disney characters without a world dedicated to their film, or original characters created solely for Kingdom Hearts.
Notes and references