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I'm quite lost on the definitions of words like omniverse, megaverse, multiverse, meta-universe, universe, and cosmos. I used to think the Universe and the Cosmos were essentially the same things, but now I'm reading the latter is an orderly system (not sure what's meant by that).

Can somebody help me understand what they each represent? In 1980, Carl Sagan said "The cosmos is all that is, or ever was, or ever will be." This sounds like the Cosmos would be the greatest of all and all else would fall into that. Would it be accurate, for example, to say Cosmos > Multiverse > Universe? What about the other things I mentioned earlier?

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    $\begingroup$ I think multiverse is a semantic trap. If multiple Universes would exist, for example in the form of alternate Minkowsky-Universes in the Penrose-diagrams, then they can interact or not. If they can't interact, then other Universes are un-physical. If they can interact, then the Universe (in its phylosophical meaning) is essentially the sum of all of them. | Cosmos is that big black on the night sky. $\endgroup$
    – peterh
    Commented Apr 3, 2018 at 4:02
  • $\begingroup$ When saying "Universe" we often mean "observable Universe". The term multiverse is a product of evolving terminology: After grasping what "Universe" / everything means (in most cases our 4D spacetime and everything in it, except maybe BH), it is straightforward to ask "Is there more?" and you quickly arrive at the multiverse. Insisting that the multiverse should be included in the term Universe is nitpicking to me, fix the term "planetary nebula" first ;-) $\endgroup$
    – Hannes
    Commented Apr 3, 2018 at 8:04

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"Cosmos" is the English cognate of the ancient Greek word which refers to the entirety of existence. It is equivalent to the Latin-derived word "universe;" the only difference between the two is the language of origin.

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