Prime Material Plane

The Material Plane, sometimes called simply the Prime, is equidistant between the Inner Planes and the Outer Planes. As such, it is equal parts matter and spirit, equal parts substance and belief, a place where the elements of the Inner Planes and the alignments of the Outer Planes exist in rough balance. The Prime touches both the Astral and Ethereal planes. The Material Plane is home to the D&D campaign worlds, such as Krynn, Oerth, Toril, Aebrynis, and Athas.

This is the plane where all the clueless are born, if you believe the inhabitants of the Outer Planes. Of course, primes point out that planars can be clueless too.

The worlds of the Material Plane are surrounded by crystal spheres. The crystal sphere is the night sky itself. It looks solid, right? In a fantastic universe, it is
solid. There are stars and constellations embedded in it, and inside
are the planets. You can literally touch the sky, or kiss it.

The Phlogiston, or Flow, is the chaos outside Creation. It's the region
the powers that be haven't gotten around to making yet. Some say they fear it, and build the crystal spheres to keep the Phlogiston at bay. Others think the crystal spheres precede the powers, created long before by the overpowers or even older beings. The Flow has currents, making travel between the various spheres in magical ships relatively swift.

Crystal spheres and phlogiston are features of the Material Plane only; Outer, Inner, and transitive planes do not have them.

Sources

Planescape Campaign Setting
Spelljammer Campaign Setting