Worldship (sometimes Craftworld) - a huge vehicle, mostly capable to self-defence. His primary goal is keeping survived many groups. They was called not just for fun, there are as huge as planetoid or even as our Earth. Popularized in pop-culture and fantastic by Warhammer 40,000 universe, where they were homes of Eldar race and where they could live in peace.
Worldships in fiction
Worldship (SPORE Saga) - a huge spaceships capable to hide and fight, where without problems people exiled from their planets or those, which lost their homeworld by war or cataclysm can live. Using mainly by Serrigans to hide before the space and to plan the conquering of all universe.
Worldship (Postconjuction) - massive vehicles where all civilizations can live. Thanks for them factions escaped before Conjuction. In the world after cataclysm founded and restored after 20 thousand years, thanks by exploration of techvaults.