The Outer Planes is a term from Dungeons & Dragons introduced in Gygax's Dungeon Master's Guide. It is a way of collectively referring to the various alignment-based afterlife planes. These planes exist "outside" of the Inner Planes.
Planescape, 2e's official setting of the Planes, at first treated these places as the most important section of the multiverse. Later Planescape product, like the third and last Monstrous Appendix, fleshed out the inner planes more, and retconned the sheer importance of the outer planes into a bias common to the residents of Sigil.
The Outer Planes can be divided by general alignment category into the Upper Planes (Good), Middle Planes (Neutral) and Lower Planes (Evil). Alternatively, they can be divided into the Planes of Law, Chaos and Conflict (Ethical Neutrality), with the Outlands as a separate plane in the center.
The outer planes simplified by Order of the Stick
|The Cosmology of Planescape|
|Inner Planes||Ethereal Plane||Prime Material||Astral Plane||Outer Planes|
|Elemental Planes||Energy Planes||Demiplane of Dread||Plane of Shadow||Plane of Mirrors|
|World Serpent Inn||Tu'narath||Sigil||Demiplanes||Ordial Plane?|