The Materium, or realspace, is the four-dimensional space-time realm where corporeal beings exist. It should not be confused with the Prime Reality -- while the Prime Reality is part of the Materium, the Materium includes various alternate timelines, parallel universes. Subspace realms, such as the Void and the Warp are not part of the Materium (although the term the Immaterium refers specifically to Warp Space.) Unrealized realities are neither part of the Materium or the Immaterium.
The Materium is bound by laws of physics which make travel through the Materium generally slow, without accessing subspace to travel more rapidly. There are areas of the Milky Way where the Materium and the Immaterium are not divided and bleed into each other. These rifts include the Eye of Terror and the Maelstrom. There are also areas of the galaxy where subspace has been destroyed, and faster-than-light travel is not possible.
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The Materium is derived from Warhammer 40,000. For canon information please visit the Warhammer 40,000 Wiki.