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 the 2040s, the generation ship SS Valencia left on a 90 year journey to Iota Persei IV. (Star Trek: Shadowstar Station: "Last Night I Dreamed Someone Loved Me")
In 2154, the Enterprise was sent back in time, to 2037, due to the presence of a Kovaalan particle wake (ENT: "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).
Kohlar's Klingon D7-class battle cruiser was a generation ship dedicated to finding the kuvah'magh. (VOY: "Prophecy")
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)