Transitive Plane
A Transitive Plane is a term introduced in the Planescape setting for Dungeons & Dragons. In a nutshell, it refers to a plane that touches upon multiple other planes, allowing it to serve as a sort of super-highway between dimensions. Transitive Planes have traditionally been described as rather bland; they're more a way to get you from point A to point B than a place to explore in their own right.
The iconic trinity of Transitive Planes in D&D are the Astral Plane, the Ethereal Plane, and the Plane of Shadow.
Some would argue that, if only by technicality, Sigil is effectively a Transitive Plane, seeing as how it's a self-contained demiplane that is defined by housing portals to every possible location in the multiverse - even in the World Axis, Sigil's still a big deal because it offers precision and expedience; you might be able to sail the Astral Sea to Baator, but a quick stop to Sigil will let you pop up right in Asmodeus' bathroom if you know where to look.
In the World Axis cosmology, the Underdark is technically a transitive plane, as one can freely cross back and forth between the Prime Material, Feywild and Shadowfell just by walking along the King's Road, to say nothing of the plethora of portals to the Elemental Chaos and possibly even the Far Realm in the Deeps.