The ISS Enterprise was the Terran Empire counterpart of Kirk's starship, a ship run by terror and assassination, which Kirk and three of his officers boarded by accident in 2267.
The ISS Enterprise appears in the original series episode Mirror, Mirror. A transporter fault during an ion storm exchanges Kirk, McCoy, Scott and Uhura with their counterparts from a parallel universe. They find themselves on a ship where officers advance by murder, discipline is enforced with the agoniser, and the captain is under orders to destroy the Halkans for refusing to hand over their dilithium.
Kirk delays the attack while Scott works on a way home. He is helped by the mirror Spock, who is bearded, ruthless but honourable, and by Marlena Moreau, who shows him the Tantalus field, the secret weapon with which the other Kirk removes his enemies.
Before leaving, Kirk urges Spock to overthrow an empire that cannot last. Deep Space Nine later reveals that Spock did.