Alternative warp
From Memory Alpha, the free Star Trek reference
An alternative warp is a type of negative magnetic corridor where the two parallel universes, namely the matter and antimatter universes, meet. It acts as a kind of safety valve, preventing the two universes from coming into conjunction, an event that would destroy both universes.
Thousands of years ago, inhabitants of a now unnamed world in the antimatter universe discovered a mechanical means of slipping through the corridor into the matter universe. One native, Anti-Lazarus, made this trip. His counterpart from the matter universe, also named Lazarus, discovered his existence. Unable to tolerate the idea of a duplicate, he went insane, pursuing Anti-Lazarus through space and time, eventually arriving in 2267. In that year, their transits through the corridor caused a cosmic "winking" effect that engendered much alarm throughout the Federation.
Captain Kirk of the USS Enterprise was assigned to investigate. After speaking with both versions of Lazarus, he discovered the truth. He forced Lazarus into the corridor, where Anti-Lazarus waited. Anti-Lazarus held Lazarus in the corridor while Kirk destroyed his ship, which also destroyed Anti-Lazarus' ship and sealed the corridor, leaving both men trapped within for the rest of eternity. (TOS: "The Alternative Factor")
