The Defiant is a prototype ship that was seen in Star Trek: Deep Space Nine. Benjamin Sisko was in charge of the design and construction of the ship while at Utopia Planitia Fleet Yards. The Defiant was seen as a significant change in philosophy when it came to the design of this ship. Generally Starfleet built exploratory ships that had the capability to defend themselves, where as the Defiant was designed from the beginning to be a combat ship. This design choice left little room for creature comforts. Crew quarters were small in comparison to other ships like the Enterprise.
It was also the first Starfleet ship to use a cloaking device regularly. Though the device was on loan from the Romulans. During it's service it was stationed at Deep Space Nine, where it was under the command of Benjamin Sisko and helped defend against the Dominion, who came from the Gamma Quadrant.
There were two ships of this design that carried the Defiant name. The original defiant was destroyed while fighting in the Gamma Quadrant. A second was built to replace the original one. Other ships were made after the Defiant's design. A few of there names areMoscow, Valiant and Sao Paulo.
Besides the series the Defiant was also seen in the movie Star Trek: First Contact where it was under the command of Captain Worf, a Klingon.