A multiverse is the theory of a conjectured set of multiple possible universes, including ours, which make up reality. These universes are sometimes called parallel universes. A number of different versions have been considered.
The term "multiverse" was coined in 1895 by psychologist William James as a philosophical concept.
The cosmological multiverse
The cosmological multiverse tries to explain why the universe we observe i.e. ''our universe'' seems so welcoming to the emergence of life. Even small changes to the way physics works would make life impossible. In a multiverse a vast number of universes are randomly created and some happen to favour life emerging there. Many inhospitable universes would also have been created, but there would be no life there to observe their existence.
The quantum multiverse
The quantum multiverse is another version in which our universe splits into alternative futures with every quantum event. This is called many-worlds interpretation of quantum mechanics.