Mirror Universe
(coexisting on another dimensional plane)
For additional meanings of "Crossover", please see Crossover.

Crossover was the term used in the mirror universe to describe the passage from the prime universe to their universe.

According to Intendant Kira Nerys, "After the first crossover, we were afraid that others might come to interfere in our affairs. It was decided then that if it ever happened again, we would promptly dispose of anyone who appeared from your side." Later, Kira explained that because the first crossover involved a transporter, "all transporters were redesigned afterwards to make sure the same thing would never happen again." (DS9: "Crossover")

The events of "Mirror, Mirror" are actually the fifth known crossover, after the arrival of the USS Defiant in the mirror universe through an interphasic rift, Gabriel Lorca accidentally transporting to the prime universe, Lorca's use of the USS Discovery's spore drive to jump back to mirror universe, and the return of Discovery to the prime universe.

Years later, when Quark and Rom were imprisoned in the mirror universe, Quark attempted to demean the Elim Garak of that universe by stating that "[i]f you were smart, you'd crossover to our universe and get some pointers from our Garak. When it comes to torture, assassination and sabotage, you could learn a lot from him." (DS9: "The Emperor's New Cloak")