Astral Plane
The Astral Plane is one of the Inner Planes of the Great Wheel cosmology - in fact, it stands opposed to Prime Material and/or Ethereal Plane in that it is the outermost of the Inner Planes, standing as the final buffer between the Elemental Planes and the Outer Planes beyond.
The Astral Plane is, for the most part, a dreary and empty expanse of nothingness, broken up upon by sporadic landmasses, the petrified corpses of dead gods, the gargantuan worm-like "astral vortexes", and color-based portals to various planes.
There are a handful of sapient races indigenous to the Astral Plane, but the most well-known are a bunch of immigrants, the Githyanki. Living creatures don't age here, so the githyanki need colonies in the prime material plane for their eggs to hatch and their children to grow up.