Mirror Universe
(coexisting on another dimensional plane)
ISS Enterprise (NX-01)

ISS Enterprise

For the space station abbreviated as ISS, please see International Space Station.

ISS, short for Imperial Star Ship, was the prefix designation given to space-faring ships in the Terran Empire registry and in the Terran Resistance Forces in the mirror universe. (ENT: "In a Mirror, Darkly, Part II")

The ISS Avenger dedication plaque is where the meaning for ISS comes from, as in Imperial Starship Avenger. This "mirrors" the meaning of USS, United Starship.

The use of ISS as a prefix for Imperial ships was already in use by the 22nd century, where ships of the NX-class were designated with the ISS prefix. (ENT: "In a Mirror, Darkly", "In a Mirror, Darkly, Part II")

The Terran Empire was still using the ISS prefix for starships in the 23rd century. (TOS: "Mirror, Mirror"; DIS: "Despite Yourself")

After the fall of the Terran Empire to the Klingon-Cardassian Alliance, the use of the ISS prefix wasn't used on vessels like the Terran raider, but was resumed when the Terran Rebellion built a Defiant-class starship based on specifications "acquired" from the prime universe. (DS9: "Through the Looking Glass", "Shattered Mirror", "The Emperor's New Cloak")