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.
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.
 Sea of the Skies
The vast space between the worlds is known as the "Sea of the Skies" (異空の海 Isora no Umi ), "Star Ocean" (星の大海 Hoshi no Taikai ), or "sea between worlds". Ventus, Aqua, and Terra travel this space using their Keyblade Glider while Sora, Donald, Goofy and Mickey use a Gummi Ship.
Travel between the worlds is normally impossible. The worlds are all protected by dimensional barriers that prevent outsiders from entering for the most part. However, these barriers can be broken, resulting in a meteor shower in that world.
In Kingdom Hearts and Kingdom Hearts II, 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. In Kingdom Hearts: Chain of Memories, there are no Gummi Ships; the worlds Sora visits are recreations from his memories from the first game brought to life on each floor of Castle Oblivion.
Four worlds can only be accessed by entering through other worlds and do not directly require use of a Gummi Ship:
 Methods of travel
Currently, there are five known ways to travel between the worlds:
Certain characters, such as Ansem, Seeker of Darkness and Young Xehanort, are capable of traversing through time, although the exact means of travel are still unknown. However, as stated by Xehanort, there are many limitations on time travel in this way. First, the person must "dispose of their body". Second, the person traveling must have existed in some form in the time period they wish to go to. Third, the person can only "move in one direction with the flow of time", i.e. continually moving forward into the future or backward into the past. In addition, the individual is not allowed to alter the events of that point in time. Lastly, the traveler and all those displaced from their original time will eventually return to their respective periods, with all memories of their time travel erased.
In the Kingdom Hearts games, each Star is the fragment of one specific world. Should a star stop shining, that would mean that 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.
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 to the Realm of Light is called the Door to Light (光の扉 Hikari no Tobira ). It 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 one reason could be, as Riku pointed out at the end of the game, the light within Sora's heart, which was brought out by Kairi's Letter.
 Realm of Darkness
The Realm of Darkness (闇の世界 Yami no Sekai ) is a realm made up of Darkness in people in every world. As with the Realm of Light, this realm has a Keyblade, Kingdom Key D, which chose King Mickey as its wielder. Kingdom Hearts is found here. This is where the most powerful Heartless can be found, as this is also the home of all the Heartless. Sora fought Ansem, Seeker of Darkness here in Kingdom Hearts. Aqua also ended up trapped here by the end of Kingdom Hearts Birth by Sleep due to saving the possessed Terra from being swallowed, effectively taking his place. Ansem the Wise ends up trapped here a second time, but lacks the way to escape like before.
 Door to Darkness
The Door to Darkness (闇の扉 Yami no Tobira ) is the dark counterpart of the Door to Light. It was shown to lie within the End of the World in Kingdom Hearts. Beyond the door is the Realm of Darkness where Kingdom Hearts, the heart of all worlds, resides.
 Realm Between
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.
 Sleeping Worlds
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 was 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.
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 for the most part hold true to the movies and cartoons they are based on. Often 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.
 World characteristics
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 movies 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 2004's Mickey, Donald, Goofy: The Three Musketeers, the source for Country of the Musketeers. 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.
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 character without a world dedicated to their film, or original characters created just for Kingdom Hearts.
 Notes and references