In the context of Games Workshop's popular Warhammer and Warhammer 40,000 (Warhammer 40k), an alternative universe interacting with our own at certain key areas of overlap (for example the soul of any sentient creature).
It can be better viewed as a set of highly curved dimensions, that bisect our normal space-time at places.
The warp is affected by the thoughts of all creatures. If a species becomes evil, it affects the warp. This can lead to the creation of Chaos Gods, who live in the Warp.