The Prime Material Plane is a term used in Dungeons & Dragons to refer to the physical universe as a whole, in comparison to more fantastical and magical realms such as the Astral Sea, Elemental Chaos or Outer Planes. It essentially functions like our own real-world universe; an enormous mass of space with stars, moons and planets in it, with the various campaign settings being inhabitable Earth-like planets within this universe. This makes it the most "normal" of the various planes.
Spelljammer is intimately tied to the Prime Material plane in that its nature as the "star fantasy D&D setting" makes it naturally serve as the default way for traveling between the various worlds of the Prime Material. It divides the Prime Material into three different components; planets, Wildspace and the Phlogiston.
In the World Axis cosmology, the Prime Material is also referred to simply as "The World".
|The Cosmology of the World Axis|
|The Cosmology of Planescape|
|Inner Planes||Ethereal Plane||Prime Material||Astral Plane||Outer Planes||Ordial Plane?|
| Demiplane of Dread
Plane of Shadow
| Plane of Mirrors
World Serpent Inn