This page contains information regarding Star Trek: Discovery, and thus may contain spoilers.
(coexisting on another dimensional plane)
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 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")
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")