Warhammer 40,000 - Existential Planes
Materium aka. realspace is the physical plane of existence. The usual plane of existence.
The Warp aka. Immaterium is the dimension of energy in physical form, maintained by unconsciousness of living, sentient beings. The Warp is believed to be the resting place of souls for humans and xenos alike. Chaos gods and other nasty things live here so don't stay for long.
The Webway aka. Labyrinth Dimension is an extra-dimensional space partitioned off from the Warp that is less volatile. Aeldari races use it for refuge and travel. Safe to visit but almost impossible to enter, exit and navigate.