A fictional universe created by committee, where several authors write their stories while referencing each other's works, backgrounds, characters and plotlines.
Shared universes may be completely committee based, may be uncontrolled anarchies where writers can fight out differences in conception in their fiction, may be rigidly controlled by a single editor who lets others play in his sandbox, or anything in between. Often, Round Robin type stories develop where one writer writes a chapter, followed by the next, and so on, and so on....