Limbo is the traditional name for dbref #0, a Room. Limbo is part of the minimal database. Historically, like Wizard (#1), Limbo(#0) was bestowed with special properties.
On PennMUSH this room is initially called Room Zero. The special properties it typically has can be moved to any other room by changing the PennMUSH configuration.
On TinyMUX these properties have been diminished over time, Limbo is now treated like any other room, and any unique behavior related to Limbo is considered a bug.