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?