Star Fort

A Star Fort is a type of void craft. They're basically space stations and can be anywhere as "small" as an Imperial Navy battleship, to as large as a planetary moon and even a planet itself. While a good chunk of them are static defenses, the 40K universe also has several mobile versions of these that either act as mobile operating bases, supply stations, an absurdly large superweapon, or a giant ball of guns.

The Eldar Blackstone Fortresses, and the Necron World Engine are some of the examples of the biggest Star Forts in the galaxy.