A sister ship of the Enterprise that was pulled into interspace and lost with her crew in 2268.
The USS Defiant was a Constitution-class starship, a sister ship of Kirk's Enterprise. In 2268 the Enterprise found it adrift in an unexplored region of space, phasing in and out of normal space in an area later called the Tholian Web incident.
A boarding party found that the crew had killed one another, driven mad by the effects of interspace. The Defiant faded away while Kirk was still aboard, leaving him trapped between dimensions. Spock and McCoy struggled to keep order on the Enterprise, which was suffering the same effects, while Tholian ships began spinning an energy web around it. Kirk was finally recovered as the Enterprise broke free.
In the Enterprise episode In a Mirror, Darkly, the Defiant is revealed to have slipped back in time and into the mirror universe, where the Tholians captured it in 2155. The mirror Jonathan Archer seized the ship and tried to use its advanced technology to make himself emperor.