A generation starship was a starship in which more than one generation passed and developed while on board during the ship's travels through space.
In the 22nd century, the Leviathan generation ships were considered the only way to get mankind to colonise space. Unfortunately for the Leviathans, faster-than-light travel was invented at the same time that their prototypes were built. (AUDIO: Leviathan)
From circa 10,000,000 to 10,000,700, the Ark, which travelled from Earth to Refusis II, was a generation starship, as numerous generations lived and died during the 700 year journey. (TV: The Ark)
The Amandala was a generation ship which was set on a course to Galaxy Eight that the Eighth Doctor visited. (PROSE: Reversal of Fortune)