Airships are large floating fortress ships used by the Koopa Troop to attack distant targets from the air or space. They tend to be mounted with numerous cannons, Bill Blasters, and other weaponry, and are heavily occupied by Bowser's forces and Princess Peach is usually kept captive in Bowser's Airship.
Super Mario Bros. 3
Airships make their first appear in Super Mario Bros. 3. One airship appears in each world (except Dark Land, under the command of one of the seven Koopalings. Completing the airship level and defeating the Koopaling inside signifies the end of that world and allows access to the next. If Mario or Luigi fails to conquer an Airship on the first attempt, it will randomly move somewhere else on the map. However, using an Anchor will prevent this from happening.
Super Mario Galaxy
Airships make much rarer appearances in Super Mario Galaxy. They are first seen in the game's intro, where Bowser uses them to storm Princess Peach's Castle and kidnap Princess Peach. Despite their presence here, airships are only seen in a handful of galaxies. They are first encountered during Good Egg Galaxy's "King Kaliente's Battle Fleet", where a pair of airships are among the obstacles Mario or Luigi must conquer on the way to the fight with King Kaliente. Space Junk Galaxy's "Kamella's Air Attack" takes place almost entirely on airships, as does Bowser Jr.'s Airship Armada. Finally, Bowser Jr. is seen keeping Peach in a personal airship during the game's final boss fight.
New Super Mario Bros. Wii
An airship is once again used to kidnap Princess Peach in New Super Mario Bros. Wii. Mario, Luigi, Blue Toad, and/or Yellow Toad spend the game attempting to chase down this airship, and it is seen dropping off a Koopaling at the beginning of each world and escaping at the end. However, in Worlds 4 and 6, the airship fails to escape in time and can be explored, leading to a fight with Bowser Jr. However, at the end of this fight, the heroes are kicked out of the airship and the chase continues. Finally, at World 8, Mario and friends finally catch up with the airship, and, after one more fight with Bowser Jr., abandon the ship as Kamek whisks Peach away to Bowser's Castle.
Super Mario Galaxy 2
Super Mario 3D Land
Being partially based on Super Mario Bros. 3, airships reappear in Super Mario 3D Land. These locations are the final levels in worlds 2, 3, 4, 6, 7, S2, S3, S4, and S6. At the end of each of them is Boom Boom, Pom Pom, or both, who must be battled to proceed to the next world.
New Super Mario Bros. U
This time, at the end of each castle, the Koopalings have an Airship. Although this is not a level, Mario and the others can only battle them in it to end the castle level. Once beaten, the Airship dramatically crashed into unknown solids.
Other airships have also appeared in the Mario games. In Super Mario 64, an airship of sorts appears in Rainbow Ride, though it is drastically different in appearance than the ships seen in other Mario games. A similar airship houses the Luigi's Engine Room board in Mario Party. Other named airships include the Koopa Cruiser, S.S. Brass, and Rainbow Ship.