A conworld is a constructed (fictional) world, such as a fantasy world, a science fiction universe, or an alternative timeline.
Conworlds can be as complex as entire fictional worlds/planets or even solar systems, to as simple as fictional single kingdoms/nations or even cities. Many conworlds feature fictional languages, and almost all include fictional peoples.
Conworlds are sometimes created just for fun and as an art form in itself, but more commonly used as a background for literature, film, TV or role-playing games. The latter application requires an especially broad and deep elaboration of the world because the characters' actions are not scripted, and it is unpredictable what they will do and where they will go next.