Materium

From Lexicanum

The Materium is the dimension where corporeal beings live; the physical plane of existence, as opposed to the Warp. The Materium abides by all the laws of physics we know today.

The Warp on the other hand is another dimension where Daemons and other Warp-entities exist. They lust to cross over to the Materium to create havoc and consume souls.

To travel through the Materium is relativly slow and the greatest achievable speed is the speed of light (lightspeed). Therefore many inteligent species travel through the warp using Warp Drives or something similar.

Certain regions within the Milky Way exist where the Warp and the Materium are 'fused'. The most infamous one is the Eye of Terror.