The Middle Planes is an unofficial term from Dungeons & Dragons introduced in the Planescape setting. It is a way of collectively referring to the three planes that are characterized by their prominent Neutral alignment as on the standard planar "map" these make up the center "spokes" of the Great Wheel, standing between the Upper Planes and the Lower Planes.
At a casual glance, the Middle Planes are the smallest planar grouping, having only three planes to call their own;
- The Clockwork Nirvana of Mechanus (Lawful Neutral)
- The Concordant Domain of the Outlands (True Neutral)
- The Everchanging Chaos of Limbo (Chaotic Neutral)
And, officially, this is the case. However, some would argue that the planes of Arcadia, Ysgard, Acheron and Pandemonium should be classified as Middle Planes rather than Upper or Lower, as they represent the "border regions" between Lawful (Arcadia & Acheron) or Chaotic (Ysgard, Pandemonium) Neutrality and Lawful/Chaotic Good (Arcadia/Ysgard) or Evil (Acheron/Pandemonium). Still, even though this is not the official ruling on the subject, fans and in-universe sages will continue to fight about the topic for all eternity.