A generation ship is a theoretical starship that would travel much slower than light across great distances between stars. Since such a ship might take thousands or tens of thousands of years to reach even nearby stars, the original occupants would die during the journey, leaving their descendants to continue traveling.
In order to assure genetic diversity during a centuries-long trip, any generation starship would require on the order of thousands of inhabitants. Additionally, the ship would have to be almost entirely self-sustaining (see Biosphere, life support) so as to provide food, air, and water for everyone on board. It must also have extraordinarily reliable systems that would not fail even over long periods of time, or alternately that could be repaired by the ship's inhabitants if it did.
Generation ships are often found in science fiction stories. A common theme is that inhabitants of a generation ship have forgotten that they are on a ship at all, and believe their ship to be the entire universe.
Some consider that planets with life are generation ships. For example, Spaceship Earth.
In contrast, see sleeper ship.
