A generation ship is a type of starship dedicated to long journeys using more than one generation of the crew during the journey to arrive at the destination.
In 2154, the Enterprise was sent back in time, to 2037, due to the presence of a Kovaalan particle wake (Star Trek: Enterprise:E²). This happened twice; the first time, the ship was sent to April 11, 2037, and Captain Jonathan Archer declared the ship to be generational on April 14 (Interphases:Reflections Down a Corridor). The second time, the ship was sent to July 14, 2037 and the captain did not need to declare that the ship had become generational (Interphases:Everybody Knows This is Nowhere).
In the late-2370s, the Romulan Star Empire launched the T'kairin-class generational cruiser, a specialised starbird designed to undertake decade long missions. (Ship Recognition Manual, Volume 5: Starships of the Romulan Star Empire)