What is a multiverse?

Some physicists say the multiverse is not a legitimate topic of scientific inquiry.[15] Concerns have been raised about whether attempts to exempt the multiverse from experimental verification could erode public confidence in science and ultimately damage the study of fundamental physics.[16] Some have argued that the multiverse is a philosophical notion rather than a scientific hypothesis because it cannot be empirically falsified. The ability to disprove a theory by means of scientific experiment is a critical criterion of the accepted scientific method.[17] Paul Steinhardt has famously argued that no experiment can rule out a theory if the theory provides for all possible outcomes.[18]

While there is nothing scientific around the term multiverse, the term has been popularized by science fiction, and conveys the idea that there are many iterations of universes that are tied together, but each in it's own way unique