Collective unconsciousness was a theorized state of mind, explained as essentially an unconscious variant of the Collective consciousness that existed in for example the Borg.
In 2374, the crew of the USS Voyager encountered a Delta Quadrant species whose reality was a shared dream, linked together by a neurogenic field. Upon the beings inducing somnolence in the Voyager crew using a neurogenic transmitter, the crew found themselves in what became clear was a shared dream, they theorized that this was made possible by inducing a state of collective unconsciousness. (VOY: "Waking Moments")