The USS Defiant was a starship from a parallel universe first captured by the Tholians and then stolen from them by the Terran Empire. It would later be used by the Empire to crush the Anti-Terran Rebellion.
The ship was constructed in the mirror universe by a Human dominated alliance known as the United Federation of Planets. The Tholians created an interphasic rift, a doorway to the mirror universe, and lured the Defiant into this universe to capture it. Once captured, it was discovered through quantum dating a piece of the hull that the rift had created a link not only to another universe, but also to another time in that universe. The ship was very advanced, compared to the human technologies of the time. Jonathan Archer aquired information about the ship from humanoid laborers employed by the Tholians, and led a successful mission to steal the ship. Although the ISS Enterprise, flagship of the Empire, was destroyed in the intense fight, the Terrans had won a significant prize. The Anti-Terran Rebellion was easily crushed by the superior ship, and Hoshi Sato eventually used it in a coup that raised her to Empress, after she betrayed and murdered Archer.