The meaning of a word derives from its usage. It is fair to say that everything that came out of our most recent Big Bang is its own self- contained universe defined by its own unique space-time and set of natural laws. There are some theories that suggest that other universes can exist that are completely separate from our own, and not detectable by any conceivable experiment either now, or in the infinite future. Multiple universe just refers to this possibility since these other universes have their own separate space-times which have nothing to do with ours.