1. A universe, especially one of an infinite set of identical universes, hypothesized in some interpretations of quantum mechanics to exist along with our own universe, obeying the same physical laws, but each in a different quantum mechanical state with no possibility for the transfer of information between universes.

2. A universe hypothesized in some cosmological models to exist along with our own universe, possibly obeying different physical laws and having the potential for the transfer of information between universes.

3. A universe in a work of fiction that exists alongside another universe in the same work or series of works, with each universe often having the same characters living in different circumstances.