Definition: The term to designate the existence of many worlds or universes. Contrary to just one world, a uni-verse, there are many worlds, a multi-verse.
More about the term: The multiverse is not monolithic but it is modeled after the contemporary understanding of an inflationary model of the beginning of this universe suggesting a plurality of worlds. Max Tegmark has championed the most prominent versions of the multiverse. There are four levels of the multiverse.
- Level One: The level one is, for the most part, more space beyond the observable universe. So, theoretically, if we were to go to the “edge” of the universe there would be more space. Having this model as a version of the multiverse may be misleading because there is still only one volume, landscape, or system involved. A generic prediction of cosmological inflation is an infinite space, which contains Hubble volumes (what we see in our universe) realizing in all conditions—including an identical copy of each of us about 10^10^29 meters away.
- Level Two: This level is typically associated with other bubble universes spawning from a cosmic landscape and slow-roll inflation. This version predicts that different regions of space can exhibit different laws of physics (physical constants, dimensionality, particle content, etc.) corresponding to different localities and a landscape of possibilities. Imagine the multiverse has a bathtub filled with tiny bubbles. Each bubble in this larger system (the bathtub) is a single universe. Or, imagine a pot of boiling water. The bubbles arise from the bottom of the pot analogous to the way inflationary cosmology works. These other domains (or bubble universes) are nearly infinitely far away in the sense that we could never get there even if we traveled faster than the speed of light (due to the constant stretching of space and creation of more volume). It may, however, not be the case that there is an infinite set of universes. Andrei Linde and Vitaly Vanchurin have argued that the way slow-roll inflation works it could only produce a finite number of universes. Hence, they propose that there are approximately 10^10^10^7 universes.
- Level Three: The third level is particular to certain interpretations of quantum mechanics such as Hugh Everett’s Many Worlds Interpretation. It is a mathematically simple model in support of unitary physics. Everything that can happen in the particle realm actually does happen. Observers would only view their level one multiverse, but the process of decoherence—which mimics wave function collapse while preserving unitary physics—prevents them from seeing the level three parallel copies of themselves.
- Level Four: The fourth level is the all-encompassing version where mathematical existence is equivalent to physical existence. Mathematical structures are physically real and the entire human language we use to describe it is merely a useful approximation for describing our subjective perceptions. Other mathematical structures give different fundamental equations of physics for every region of reality. This would be Plato’s ideal reality.
Example of use: Some physicists posit the existence of a multiverse because they believe that the parameters for physical constants and values can change from universe to universe. The multiverse may be used as an attempt to circumvent this problem.
 Max Tegmark, “The Multiverse Hierarchy,” Cornell University Library arXiv, arXiv:0905.1283v1, (accessed March 15, 2011).
 When Tegmark refers to an “identical” copy he simply refers to a similar copy. There is a genuine ontological distinction. Tegmark, 2.
 Ibid., 7. Additionally, there have been good scientific evidence suggesting observational grounds for inflation. Researchers have taken the 7-year WMAP data and applied certain algorithms to pick up traces of thermal fluctuations in the early universe. What they found were traces of what could be bubble collisions of the edges of our universe with another universe. Stephen Feeney, et al., “First Observational Tests of Eternal Inflation: Analysis Methods and WMAP 7-year Results,” Cornell University Library arXiv, arXiv:1012.3667v2 (accessed March 16, 2011).
 Andrei Linde and Vitaly Vanchurin, “How Many Universes are in the Multiverse?” Cornell University Library arXiv, arXiv:0919.1589v2 (accessed March 15, 2011).
 Tegmark, 10.
 Tegmark, 2, 12-13.