Rama - The Generation Ship

A generation starship, is a hypothetical type of interstellar ark starship that travels at sub-light speed. Since such a ship might take centuries to reach nearby stars, the original occupants of a generation ship would grow old & die, leaving their descendants to continue traveling. The ship should thus be quite massive as it should provide an livable ecosystem. 

Rama, the ship in Arthur Clarke's book, is the archietypical representation of the concept, and a major cultural influence.