In my travels, I've heard the term "multiverse" mean a multitude of things. Therefore, I believe a clarification of terms is in order: what, precisely, do I mean when I say "the Multiverse?"
The Multiverse (which shall, from this point onward, be denoted by capitalization) is, to put it simply, the collection of all potential universes. To further simplify, it is the collection of everything that is, or could be, real.
Many scratch their heads at this point, and to them I ask a simple question: what is real?
When we process reality, we use our own minds to process what, precisely, is real and what is false. We take into account our own experiences and perceptions, and through that knowledge develop our own conceptions on what is real and what is not. We know that the talking pastel horses on our television screen are figments of someone's imagination because we have never seen them, and have been informed by a multitude of reliable sources that there have been no sightings of talking pastel horses in the world. But what would happen if someone could show you a talking pastel horse? Was it real before you witnessed it, or did it just come into being when you learned of its existence? Or what if one of those horses told another horse of large, hairless ape-like creatures who were similarly capable of speech? Would they be simply "wrong" in believing such a being doesn't exist?
In truth, the Multiverse is a complex system of reality and un-reality, working in tandem to create a multitude of seperate, distinct timelines. While one world may discuss the three goddesses and the dark lord of all and their constant struggle, another world may dismiss that all as "a load of drek," matters found only in an archaic game that children once cherished in the distant past. While one world rides on the backs of giants, another spends its days weaving under their feet, praying to the gods to not be crushed or - worse - sent flying to the stars by their mighty swing. Some speaks of parallel planes attributed to the basic essences of magic; others speak of worlds beyond the stars, each with their own history and populace.
Of curious note, despite all variance between the vast multitude of worlds, many similarities can be picked up within them. Some theorize that this is due a hidden Multiversal law strung throughout the realities; others claim that all are descendent of a single world, known simply as the Prime World. Some believe it to be the work of a single Creator, while still others believe it to be a cosmic coincidence of sorts. No one I have talked to can say for certain, though some discuss their own claims.
Also of curious note, the Multiverse contains a multitude of "multiverses" (lower case) which each contain their own bundle of realities. To make matters even more baffling, these multiverses, depending on how extensive they are can contain their own versions of the Multiverse. "Going too deep" is a frequent issue encountered by novice explorers, and it is imperative that you keep track of your precise path through these areas, if possible. Trust me when I say being lost in such a manner is not a pleasant experience.
In closing, the Multiverse is simply a way to denote the infinite realities that exist. Any further discussion on this topic will use this definition, and it is an important definition to keep in mind.
