Constitution-class 23rd-century Federation starship, registry no. NCC-1764. Caught in a spatial interphase in 2268, near Tholian territory, the U.S.S. Defiant was lost between dimensions for several hours, until it finally faded forever into interspace.
Due to adverse affects on humanoid neurophysiology, caused by the interphased space, the crew of the Defiant went insane.
In the Mirror Universe, circa 2155, NCC-1764 was kept inside a secret Tholian spacedock facility. The Mirror Universe's Jonathan Archer commandeered the vessel, believing its future design — 100 years ahead of the I.S.S. Enterprise — could turn the tide in the Empire's campaign. Archer used the ship's formidable weaponry, which included aft phasers and torpedoes, to defend the I.S.S. Avenger from enemy attack.
The Defiant was set on a course for Earth, for Empire HQ, when Archer's concubine, Hoshi Sato, fatally posioned him. Now in command of the starship, Empress Sato assumed leadership of the Terran Empire, demanded all to surrender under her rule, and had more than enough firepower — courtesy of the Defiant — to back her up.