The Prime Material Plane is the plane in which Ord and the surrounding cosmos exists. The phrase "Prime Material Plane," or simply the "Prime," is generally used by beings from other planes in the Ord cosmology (notably Concordant Opposition), and "prime" is used by the same beings to refer to the inhabitants of Ord.
There is supposedly an infinite number of systems, among them the Prime Material Plane, Realmspace, Greyspace, Krynnspace, and the system in which Earth is located. The line between material space and the phlogiston can not normally be seen. Each system is separated from one another by ethereal crystal spheres and travel between is difficult without powerful magic or by the use of special ships.