A Secret World is a location that is not within a game's normal guidelines. These areas were first found in Metroid using the Door Glitch. Areas were discovered using this glitch that were not connected to other areas but existed nonetheless. Players called these areas Secret Worlds, and the name stuck. In Metroid II: The Return of Samus, use of the Select Glitch allows players to leave the normal areas and enter new places. These areas are not programmed to be interacted with at all and are filled with glitch pieces of sprites. These areas are called Secret Areas.
In the Metroid Prime games, players can leave rooms through holes. Outside of the area, players encounter a void from which they can look in on the area from whence they came. They can move forward back, left, and right, but they cannot jump. If a player makes Samus become the Morph Ball, Samus will fall endlessly. Despite the obvious differences between them all, each of these areas are called Secret Worlds.