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.