I have chosen to adopt the term "Multiverse" to define a large set of universes which can or should be grouped together. It should be mentioned that many people use this term interchangeably with the term "Omniverse." (Please see blog entries titled "Our House" and "MWI Diagram" for further explanation.) The term Multiverse applies to certain universes that share certain traits, allowing for a logical and reasonable grouping to form. All universes with the same initial conditions and corresponding laws would group nicely together.
[Some people will disagree with the use of terms like Omniverse and multiverse, saying that the term "universe" "uni" meaning one and "verse" meaning word, implies that that "one-word" is as all inclusive a term as we need to use. This view and terminology is fine, unless you want to explain the larger concepts of MWI. An infinite universe would work just fine as long as it was truly infinite "enough" so that all possible outcomes and all possible interactions could take place somewhere or somewhen within that one universe.]
These similar universes need not appear so much the same (at least considering their appearance) some planets might be missing or out of place, some might be unusual additions. The laws, and the way that everything functioned would be very much the same. It can be said (with more than a little certainty) that the initial conditions and the act of the universe's creation should contribute that universe's laws and the way that it functions all at the same time. A twin or sibling universe to our own might look like our universe down to even the species of grass growing on (what might be your "twin's" front lawn,) but subtle and not so subtle changes would inevitably occur as random outcomes gave way to alternate historical time-lines. Hitler might have teamed up with Mexico and South American forces, winning the second world war, or maybe WWII never even occurred! I have noted before that some universes should not hold our same initial conditions, and would thus be grouped into a different set of universes outside of our Multiverse. It is important to remember though, that this designation is purely one of categorization. There is no reason why these universes (with different initial conditions and laws) could not exist right next to or even right on top of one another, possibly sharing the same physical space or containing gateways in and out of one another. Many current cosmologists view the MWI model to suggest that each universe is contained within some kind of cosmic "soap bubble." Since we have not even the means to suggest how we can leave our own universe, or manage to see it from a greater vantage point, this "bubble concept" will have to remain a logical supposition. In the blog entitled "...world without end..." I discussed the idea that whatever the state of the greater cosmos may be, the end result must logically be an infinite in time and space larger "whole." This all inclusive, all containing "whole" I refer to as the Omniverse, a staggering concept that contains (as the picture in MWI diagram shows) all worlds in all universes, across all time and space. This Omniverse is infinite in every way. Logic tells us that beyond the boundary of the larger container lies something, that something must be contained by something else, and so on and so on, ad infinitum. An ancient philosopher once asked, if a man threw a spear, and were able to continue to do so on and on, where would the end be? If that spear were a spaceship traveling in hyperspace (far faster than the speed of light) where should it end? The answer is the same. There can be no end, for the Omniverse is infinite in both every quality and every quantity, it is the sum total of all existence and all non-existence!