A parallel universe, or alternate universe, is a universe that exists separate and parallel to our own universe. A parallel universe might be on a separate dimensional plane and could be governed by different laws of physics. They could be within subspace or be a quantum reality.
- The mirror universe is one of the best known alternate universes due to the multiple contacts between beings from this universe and the 'mirror universe'. Our universe first became aware of this other universe in 2267. Contact between the two universes has led to significant changes in the mirror universe. (TOS: "Mirror, Mirror"; DS9: "Crossover", "Shattered Mirror", et.al.)
- Another quantum reality existed where the Dominion won the Dominion War. The point of divergence was Operation Return where the USS Defiant failed to prevent the Dominion's reinforcements from entering the Alpha Quadrant and was destroyed in the Bajoran wormhole. (Star Trek: The Farthest Star: "The Final Prophecy")