Imagine if there were multiple universes, where you live a different life, have different friends, work a different job, and tackle different life problems, where you don’t even have access to the internet. These multiple universes are all related to ours and they branch off from ours, and our universe is branched off of others. Within these alternative universes, our history is different, because our politics and our wars have had different outcomes than the ones we know. Additionally different forms of life, animals, plants and other species that are extinct and gone in our universe have evolved and adapted in others. In those other universes, we humans have become extinct.
This is a concept called the multiverse (or meta-universe) which is the hypothetical set of multiple possible universes (including the historical universe we consistently experience) that together comprise everything that exists and can exist: the entirety of space, time, matter, and energy as well as the physical laws and constants that describe them. The term was coined in 1895 by the American philosopher and psychologist William James. The various universes within the multiverse are sometimes called parallel universes.
The structure of the multiverse, the nature of each universe within it and the relationship between the various constituent universes, depend on the specific multiverse hypothesis considered. Multiple universes have been hypothesized in cosmology, physics, astronomy, religion, philosophy, transpersonal psychology and fiction, particularly in science fiction and fantasy. In these contexts, parallel universes are also called "alternative universes", "quantum universes", "interpenetrating dimensions", "parallel dimensions", "parallel worlds", "alternative realities", "alternative timelines", and "dimensional planes," among others.