The Nebula-class Star Destroyer, also known as the Defender-class Star Destroyer, was the largest, most powerful warship design in the New Republic's New Class Modernization Program.
Although it was only 1,040 meters long (65% of the length of an Imperial-class Star Destroyer), the Nebula-class was built to be a match for an Imperial II. Even with a structure that was a significant departure from the Imperial-class, especially the lack of a command tower, the Nebula was often mistaken for an Imperial design due to its angular lines. Its small size often lead to it being referred to as a "pocket Star Destroyer", much as the later Turbulent-class would be as well.
It was designed to be able to defeat any one enemy Star Destroyer, two heavy cruisers, or an entire line of smaller Imperial support ships. It mounted defenses strong enough to resist even some of the smaller Super Star Destroyers or analog warships.
The armament of the Nebula-class Star Destroyer consisted of 40 heavy dual turbolaser cannons, 40 turbolaser cannons, 20 ion cannons, 8 tractor beam projectors, and 8 assault concussion missile tubes, each equipped with 30 missiles. Despite the touting of far superior firepower over the Republic-class, the two had nearly similar armaments: the difference was eight concussion missile launchers for the Nebula, and two additional tractor beams for the Republic. The power generation and model of guns was different enough to make the Nebula a far superior design to the Republic. In addition, the hull armor and shield systems of the Nebula outclassed most other ships in the galaxy, including the Republic. 
The ships could carry up to sixty starfighters, normally including one assault wing made up of a mixture of K-wing bombers for precision strikes and Defender starfighter for close support. Nebulas could also host a space superiority wing consisting of hyperdrive-equipped E-wings, A-wings, or upgraded X-wings. They also carried 1,600 troops and enough consumables for five to six months, depending on configuration.
Nebula-class Star Destroyers were equipped with a Class 1 hyperdrive for faster than light travel. The design also incorporated a system of lateral thrust vents to increase the ship's maneuverability, giving it far greater agility than most large capital ships.
Following the New Class practice of using the same hull for multiple configurations, the Nebula-class Star Destroyers shared their basic spaceframe design with the Endurance-class fleet carriers.
Development of the Nebula-class Star Destroyer began shortly after the fall of Coruscant. Formally renamed as the Defender when it was made a part of the Defender design program, the Nebula-class refined the core elements of Walex Blissex's Republic-class design into a smaller, more angular hull. The Defender program was derailed by delays caused by political distractions and the campaigns of Grand Admiral Thrawn and the reborn Palpatine. When work eventually resumed the design became part of the New Class program and lent it's hull design to the Endurance-class carrier. The Nebula-class eventually entered service almost a decade after it first entered development. The first of the class to enter service was the Obi-Wan, named after one of the Rebellion's heros. The ship was commanded by Captain Whyrrryk, a female Wookiee.
Due to the process of building compact yet powerful warships of this sort, only a small number of Nebula-class ships were built. Only ten were in service c. 17 ABY, with projections of one more being launched each year until the New Class build program was completed. When the Yuuzhan Vong invaded, the few ships of the New Republic's valuable Nebula-class were kept close to the Core Worlds in reserve fleets due to political pressure. Despite this, Nebulas performed well in clashes with the extra-galactic warships once they finally did see combat. Even considering its age, the Nebula-class still represented the peak of modern, efficient Star Destroyer design during the Yuuzhan Vong War. 
At least one Defender-class Star Destroyer was still in service by the time of Darth Krayt's Galactic Empire. Pol Temm maintained the ship as part of his The Wheel defense fleet. Attempting to enforce his no-weapons rule, he entered a space battle with a Mandalorian faction called the Mandalorian Supremacists and was reportedly killed when his Defender blew up.