There is a word for everything that exists, for the sum total of all the atoms, mice, cities, forests, planets, galaxies, and galactic superclusters that exist, have ever existed, or will ever exist. That word is “universe”. It used to be “world”, but then the totality of everything got too big for the world and a new word was needed. It’s possible that a new word is needed again.
The word, if it is indeed needed, is “multiverse”.  It sounds like it means multiple totalities of everything, which wouldn’t make any sense. It doesn’t quite mean that; like world before it, the concept of a universe has become too small to contain everything. The world is what contains everything that we encounter, and — for almost all of us — our lives, beginning to end. It is also a ball of rock hurtling through empty space as it orbits the sun. There are other balls of rock hurtling through empty space orbiting the sun. There are even more balls of rock hurtling through space orbiting other suns. A world still contains us, but is too small to contain everything.
Our universe is a collection of stuff — mostly Dark Energy with some Dark Matter and a tiny bit of ‘ordinary’ matter thrown in for seasoning. The latter two are organized into web-like networks of galactic superclusters. There are also photons (light), neutrinos, black holes, and — very, very, very, very occasionally — a human. (There is some overlap in these categories.) In addition to all being in roughly the same place (give or take a few tens of billions of light-years), all of these things obey the same laws of physics.
So once upon a time, we took our previous concept of world and applied it to all the balls of rock flying through space. We did the same thing with “sun”. It was the ball of fire in the sky, the source of light and energy and life for all things. Our sun still is that, but any ball of gas shining through fusion and with planets orbiting it can now also be a sun.
And just like that we can generalize the word universe as a region, or perhaps bubble, of space governed by a particular version of what we call the laws of physics. In other universes the laws might be slightly different — electric charges might attract and repel each other with a strength that is a bit stronger or a bit weaker; or protons might weigh slightly more or spacetime might bend more easily. Or, the laws might be very different. There might be a completely new force; say something like electromagnetism that only acts on electrons and not protons. Each universe would behave slightly, or drastically, different. Most would collapse a tiny fraction of a second after forming. Some will expand so fast galaxies and stars and planets will never have time to form. A tiny few will have the right conditions to support life.
Now, what’s weird is not that we can imagine such universes, but that there is a reasonable way they could all exist, and exist within the same “totality of everything” that we do, separated not by some mystical dimensional barrier, but simply by ordinary space. Incomprehensibly large tracts of space, but ordinary space none-the-less.
This is the crux of the multiverse theory, which has become quite popular recently. Through a string of apparently disconnected theoretical developments this picture of bubble universes spread throughout a much more vast space has gone from pure fantasy to quite conceivably real. (Note the qualifier in the previous sentence. Many of the pieces required by this theory have not been empirically verified.)
 I’d nominate “cosmos” in memory of Carl Sagan, but multiverse seems to have won for now.