(coexisting on another dimensional plane)
The ISS Defiant (NX-74205) was a Terran Defiant-class warship that was in service to the Terran Resistance Forces in the latter half of the 24th century. The Defiant was constructed and launched in 2372 by the Rebellion at Terok Nor, in orbit of the planet Bajor. The design of the Defiant was based on specifications of its counterpart the USS Defiant which were "acquired" from Deep Space 9 in 2371. This warship proved of crucial importance in fighting the Klingon-Cardassian Alliance at the Battle of Terok Nor, and represented an important boost to the Terran's fighting power during the Terran Rebellion.
In 2372, shortly after the rebels captured Terok Nor, the Alliance launched a major assault fleet to take back the station, a fleet personally commanded by Regent Worf. Recognizing the danger in light of the fact that they had only a handful of raiders remaining, "Smiley" O'Brien and the other rebels began constructing the Defiant. Construction proceeded relatively smoothly and rapidly until the final testing phase. Much like her counterpart had in testing, the ship nearly shook itself apart when the warp core was used at full power.
Desperate for help, O'Brien recruited Captain Benjamin Sisko, who had overseen the construction of the original Defiant several years earlier in his universe. Sisko recommended a complete overhaul of the ship's inertial dampers to enhance structural integrity, and because the repairs were rushed due to the incoming Alliance fleet, they were completed before the battle began.
Although the small ISS Defiant was heavily outgunned by the Regent's flagship, the rebel ship inflicted major damage on the massive warship while avoiding most return fire due to the command and later piloting of Captain Sisko. Under the Defiant's heavy assault, the Regent was forced to abandon his goal of retaking Terok Nor and retreated from the battlefield. (DS9: "Shattered Mirror")
In 2375, Ezri Tigan brought both Quark and Rom from their universe with a Klingon cloaking device, which they were to install in the Regent's flagship as ransom for Grand Nagus Zek. The Defiant, under the command of "Smiley", came after them, hoping to reach the ship before the cloaking device was fully installed. Rom, however, manage to install the device, and the battle cruiser was able to track the Defiant undetected. After testing the cloak against the Defiant's sensors, the Regent ordered the ship to decloak and attack. During the battle that followed, the Regent's ship was disabled, due to sabotage Rom had inflicted during the installation of the cloaking device, and the Defiant was able to force a surrender and take the Regent prisoner. (DS9: "The Emperor's New Cloak")
Aside from the lack of a cloaking device and different internal furnishing, the ISS Defiant was virtually identical to her prime universe counterpart, proving herself one of the most powerful warships in the sector. The ship had four decks and was of a compact design. The bridge, located on deck one, incorporated the tactical, science, and operations consoles on each side with one central helm console in front of the commander's chair.
The ship was equipped with four phaser cannons, two on both the port and starboard sides, at least one aft phaser emitter, and a complement of quantum torpedoes, the latter a previously unknown technology. Together with powerful deflector shields, the Defiant was able to hold its own against several smaller Klingon Birds-of-Prey and inflict heavy damage on the larger Galor-class and Negh'Var-class battle cruisers despite never being battle-tested. (DS9: "Shattered Mirror", "The Emperor's New Cloak")
Background information Edit
This Defiant appeared with no prefix or registry number on the hull, and was simply called Defiant in dialogue. Displays seen in the ship, and stock footage used briefly in "The Emperor's New Cloak", had the prefix and registry number of the prime universe Defiant.
Lot #164 of the Las Vegas Propworx auction was the dedication plaque cover used on the bridge of the Defiant in "Shattered Mirror" and "The Emperor's New Cloak". This is where the prefix and registry number Memory Alpha uses comes from. According to the lot description, it came from the collection of Doug Drexler. 
Chronologically, this is the second time that a faction in the mirror universe has benefited from a Starfleet vessel named Defiant. The first was seen in the Star Trek: Enterprise episodes "In a Mirror, Darkly" and "In a Mirror, Darkly, Part II" in which the Constitution-class starship USS Defiant became trapped in a spatial interphase and was pulled into the mirror universe, where it was captured by the Tholians, and then later, the Terran Empire.
From the Star Trek Encyclopedia (4th ed., vol. 1, p. 196), "The Defiant's registry number, NX-74205, is in honor of Gene Roddenberry's son, our friend Rod Roddenberry who worked with us as a member of the Star Trek: Deep Space Nine art department. Rod's birthday is February 5, 1974."
The Obsidian Alliances collection of mirror universe stories has the Terran rebels breaking into two factions, with one led by mirror Bashir and mirror Zek, who take over Empok Nor and building twelve other ships at a time. However, the disgraced Intendant Kira figures out the location of this base, and with Captain Kurn they destroy the base, along with the unfinished Defiant-class ships, killing everyone except for Zek and Bashir, who are captured.