Collective unconsciousness is a state whereby several individuals can be telepathically linked together while asleep, sharing experiences and observing a single dream state from a unique point-of-view.
In 2374, the crew of the USS Voyager encountered a species in the Delta Quadrant which existed normally in a state of collective unconsciousness, telepathically linked together by a neurogenic field. Using a transmitter, the aliens amplified the field and used it to induce unconsciousness in the Voyager crew, trapping them in a collective dream. Eventually, Commander Chakotay and The Doctor were able to force the aliens to release the crew after identifying the location where the aliens slept, and threatening to destroy it with a photon torpedo. (VOY: "Waking Moments")