A drifting zone of space in which the laws of physics are in constant flux, where Voyager became trapped in 2375 and its inhabitants reached out through Chakotay's hallucinations.
Chaotic space is a rare phenomenon in which the ordinary laws of physics no longer hold steady. Sensors and navigation become useless inside it, and Seven of Nine reports that the Borg have observed it only a handful of times, losing every cube but one that entered.
In the Voyager episode The Fight, the ship is swallowed by such a region. Chakotay begins to hallucinate, his visions built around a boxing match against an opponent called Kid Chaos with the Academy groundskeeper Boothby in his corner.
The Doctor finds that aliens native to the region are trying to communicate by activating a gene for a cognitive disorder that Chakotay inherited from his grandfather. By accepting the visions he is able to understand their instructions and guide Voyager out.