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The etymology of the word universe comes from the Latin words uni, meaning one, and versus, meaning turn and is attested from Late Middle English. It suggests that the universe is 'mortal', having a lifetime, from a beginning to an end - an epoch.

This contrasts with the Indian conception where the universe is eternal, and when one epoch (yuga) ends, another begins.

One may also want to contrast this with the notion of the Multiverse (which though recently fashionable goes back to Atomists who following a principle of plenitude, posited something of a like kind and one might also wish to contrast this principle with, in one sense its opposite, the principle of parsinomy - Occams Razor) which has many causally & spatially disconnected parts, each of which is called a universe.

However, a natural definition of the universe is everything that has been, is, and will ever be. This, appears at first glance, to subsume that of the multiverse, as each part - each universe, is a thing, and has been, is or will be.

But one notices that each universe would have it own time coordinate, and one could be justified in claiming that this definition of a universe names only that part of the multiverse we have already christened as a universe.

What is the best definition of a universe and multiverse?

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There are at least two ways to approach this question. The first is the way you mention at the beginning of your post. If the word "universe" means "everything" then the word "multiverse" doesn't make sense. I can see that taking it from this linguistics perspective is be confusing.

From an astrophysics perspective, the term used is "observable universe." This is the area of existence that we can see and was caused by our Big Bang. The current theoretical understanding is that nothing before the Big Bang is observable (because time and all observation collapses into a singularity at that point), so there will never be any evidence for any theory about what happened before the Big Bang or adjacent to the Big Bang. So, many scientist choose to keep out of the speculation and to them the "observable universe" is the "universe."

Other theoretical scientists try to make theories about what is outside our universe. They reason that the Big Bang did not happen by itself and they want an alternative hypothesis to "God created it." The multiverse theory gives them that because it states that there are an infinite number of "observable universes" each created separately from the others. Inside each universe, you can only see the things inside that universe and have no way to observe other "universes. Thus, to an observer in a universe, that universe is "everything." The explanation is complicated, but at a basic level each universe is said to have sprung out of nothing by spontaneously creating equal amounts of matter and anti-matter, energy and anti-energy.

So, it depends on how you define your terms. If you are looking at this in a purely linguistic sense, then "universe" means "everything" and "multiverse" makes no sense. If you are looking at this from an astronomy perspective, then "universe" means "observable universe" and "multiverse" means "all of the individually observable universes." "Multiverse" is astrophysics jargon after all, so it is best to understand it in that context.

For more information, I highly suggest reading Stephen Hawking's (world renowned theoretical physicist) "A Brief History of Time" (and his other books) which explain these concepts in detail. Most of the information in this post can be cited from his books.

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