From ROI Wiki

A Worldship is a super massive starship, some the sites of entire planets, designed to house an entire race and be mobile as needed. The Buyyor are the most famous for using this kind of ship, and possess over a dozen in their extensive, and galaxy wide fleet. Most of the smaller worldships are capable of speeds of up to warp 4 (between 90 and 120x's the speed of light) while the largest ones are only capable of sublight speeds upwards of 1/2 of the speed of light. But, this handicap isn't much of a problem when one considers that most Buyyor worldships don't move very far at any one time. Movements between systems are rare and usually both relegated and limited to the necessities of relocation due to war or other reasons.