The multiverse theory is the hypothetical existence of a set of multiple possible universes, which includes our universe. These multiple universes are composed of everything that physically exists: the entirety of space and time, all forms of matter, energy and momentum, and the physical laws and constants that rule them. These different universes contained within the multiverse are sometimes called parallel universes. The structure of the multiverse, and the nature of each universe held within it, depend on the specific multiverse hypothesis considered.
Multiverses have been hypothesized in physics, astronomy, and philosophy. The word "multiverse" was coined in 1895 by psychologist William James.