Universe of Kingdom Hearts

Realm of Light
The Realm of Light (光の世界) is a Plane of Existence made up mostly of light and makes up almost half of the Kingdom Hearts universe, similar to the Realm of Darkness. Like the Realm of Darkness, the Realm of Light has a Keyblade, notably the Kingdom Key, which chose Sora as its wielder. Most of the game's worlds are found here, save for those in the Realm of In Between, like Twilight Town, Castle Oblivion, and The World That Never Was. It can be presumed that most of the Worlds accessible by the Gummi Ship are either within the Realm of Light, or are related to it, like Twilight Town.

In Kingdom Hearts II, one notable citation has been made by Sora regarding the Realm of Light during the scene of Sora and Riku at the "end of the road" after defeating Xemnas: ''The other side... The realm of light is safe now. Kairi, the King and the others are there.''

Door to Light
"Hey Riku.... what do you think it was, the Door To The Light?"

"This."

"This?"

"Yeah. It's always closer than you think."

- Sora and Riku at the end of Kingdom Hearts II

The door leading to the Realm of Light. It's opposite counterpart is the Door To Darkness. It was the door Sora and Riku went through while they were stuck in the Realm of Darkness. It is not sure what caused the door to appear, one reason could be, as Riku pointed out at the end of the game was 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) is a Plane of Existence made up of Darkness in people in every world. As with the Realm of Light, this realm has a Keyblade which chose King Mickey as its wielder. Kingdom Hearts is found here. This is also the home of all the Heartless, as this is where the most powerful ones can be found. Sora fought Xehanort's Heartless in Kingdom Hearts.

Corridor of Darkness
Corridors of Darkness are unpredictable pathways that interlink the worlds of the Kingdom Hearts universe. The Heartless use these extensively to reach other worlds, as do members of Organization XIII, their subordinate Nobodies, and many other characters. These pathways are formed out of Darkness, so it is unsafe for a normal person to travel them, because the constant use of these corridors would corrupt one's heart. However, the Heartless and the Nobodies don't have to worry about this, since the Heartless already have corrupted hearts, and the Nobodies do not have hearts to corrupt. However, one upside to this is that it provides a faster means of travel than the Gummi Ships.

Other characters who have used these corridors include Riku, Naminé, DiZ and even Pluto. Of these, only Riku, Naminé and DiZ have been responsible for summoning the portals, and only those lacking hearts to corrupt (Nobodies), and those who are darkness (Heartless) aren't affected, with two noted exceptions; According to Nomura in the Director's Secret Report XIII, Riku and Mickey aren't affected by darkness at all, due to some "secret similarities". Ironically, in the first Kingdom Hearts, after the faceoff in Neverland, Riku's heart was significantly weakened as he carelessly used a Corridor of Darkness to return to Hollow Bastion. Nomura goes further to state that DiZ and the Beast can successfully use the Corridors of Darkness, and perhaps open them, due to their particularly strong feelings or hatred. Furthermore, despite Sora having used the Corridors of Darkness several times before, he has not used them frequently enough to allow the Darkness to stain his heart, and the influential power of Darkness depends on the strength of a person's heart.

Anyone who has traveled in darkness for too long would show a few physical traits: their skin darkens and their eyes turn amber. This is especially true for Xehanort, whose Heartless and Nobody both have tan skin and amber eyes. In the case of Ansem the Wise, it is not so severe: he only has amber eyes.

Maleficent is also believed to travel the Corridors. Though she does not show the physical changes. This could be due to her trying to command the Darkness, instead of becoming one with it. It is also know that Pete travels the Corridors as well. He is known to be protected by the Darkness by Maleficent's magic.

In Kingdom Hearts Re:Chain of Memories, whilst DiZ gives out black cloaks to King Mickey and Riku, he says the cloaks protect the wearer from the darkness.

Dark Meridian
The Dark Meridian (referred to by Tetsuya Nomura as the Dark Coastline) is a mysterious place in Kingdom Hearts. It is first seen in "Another Side, Another Story" (Deep Dive) video. In the first cutscene of Kingdom Hearts II, Roxas on a dark, moonlit beach, where Xemnas appears. He talks to Roxas, telling him how much he looks like Sora.

It reappears in the end of Kingdom Hearts II. After defeating Xemnas, Riku and Sora end up on the Dark Meridian. Suddenly, after Sora reads a letter from Kairi, a door appears leading them to Destiny Islands.

In Kingdom Hearts 358/2 Days, Roxas converses with Xemnas at the Dark Meridian.

According to Another Report, Nomura states that the Dark Meridian is "the tip of a world", i.e. the boundary between Darkness and In Between. Sora and Riku were on the coast of the side of In Between, and were looking over the sea in the Realm of Darkness. Having said thus, the Door to Light which appeared in the ending scene is reminiscent of a quote which recurs in the Kingdom Hearts series, albeit in different words: "Deep in the Darkness, there is Light".

Finding the Box
After being saved from Saïx by Maleficent in Hollow Bastion, Sora, Donald, and Goofy are transported to the Realm of Darkness, where they encounter a man in a black cloak, later revealed to be Riku. Riku leaves a box with a picture of Hayner, Pence, Olette, and Roxas and a bar of sea-salt ice cream. In this same realm, the Keyblade reacts to the ice-cream and Sora opens a pathway. This is the first time that Sora, Donald, and Goofy are able to make their way to the Realm of Darkness in Kingdom Hearts II.

Realm of In Between
The Realm of In Between (or the Realm of Twilight) is the third Plane of Existence in the Kingdom Hearts series, and lies between the Realm of Light and Realm of Darkness.

Thus far the only known Worlds to exist in this plane are: Traverse Town, Castle Oblivion, Twilight Town and The World that Never Was. Each world tied closer to the Realm of Light or the Realm of Darkness, with the exception of Twilight Town, which exists exactly in the middle.

Yen Sid's Tower also exists in this Realm.

Realm of Nothingness
Nothing exists here and if one stays in there for too long, they will cease to exist. The only way to survive here is to remember your past.

Sora and Riku briefly entered the Realm of Nothingness during their final battle with Xemnas (i.e. in his final form).

After being banished from Hollow Bastion by Xehanort, and his apprentices, Ansem the Wise wandered into the Realm of Nothingness and became DiZ. DiZ only managed to stay alive and prevent himself from going insane by continuing to think and write.

Trivia

 * The rocks of the Dark Meridian are very similar to the rocks of the End of the World, which befits this place as Riku calls it the "end of the road".
 * With the use of cheat codes in Kingdom Hearts II, you can access the Dark Meridian and the area where you fight Xemnas the final time.