Shared Universe
Definitions
There is no formalized definition of when the appearance of fictional characters in another author's work constitutes a shared universe. Fiction in some media, such as most television programs and many comic book titles, is understood to require the contribution of multiple authors and does not by itself create a shared universe. Incidental appearances, such as that of d'Artagnan in Cyrano de Bergerac, may instead be considered literary cameo appearances. More substantial
