A shared universe is a fictional universe in which multiple independently created works are set. One or more authors may contribute works to a shared universe. The works within the universe may share characters and other story elements, with or without continuity. Shared universes can be found in literature, comic books, films, television, anime and manga and are most commonly seen in fantasy and science fiction genres.
A good example of the shared universe in action is this after-end-credits scene from the film Iron Man (2008) where Tony Stark (Robert Downey Jr) and Nick Fury (Samuel L. Jackson) cross paths for the very first time:

(after end credits)
Tony Stark: (arriving home) Evening, JARVIS!
Jarvis: (voice distorted) Welcome home, sir...
(Stark stops as he sees a figure in his living room)
Nick Fury: "I am Iron Man". You think you're the only superhero in the world? Mr. Stark, you've become part of a bigger universe. You just don't know it yet.
Tony Stark: Who the hell are you?
Nick Fury: Nick Fury. Director of SHIELD.
Tony Stark: Ah.
Nick Fury: I'm here to talk to you about the Avenger Initiative.
by The Centurion December 28, 2015
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