Definition: Modal realism is the idea that all modal possibilities are actual.
More about the term: Anything that is possible actually happens. However, modal realism is, in a sense, modally limited. The state of affairs of the non-existence of anything cannot be true if something does exist so by definition modal realism must entail ~∃!W with W being the non-existence of anything—nothing, lest it suffer the consequence of being intrinsically incoherent (~∃!W = There does not exist just one W). In order to avoid an inherent incoherence perhaps there are logically antecedent reasons to affirm ~∃!W (i.e. actuality is logically prior to possibility, which makes possibility somewhat superfluous). Under certain multiverse scenarios different regions of space will exhibit different effective laws of physics (i.e. difference constants, dimensionality, particle content, relation of information, information propagation, etc.) corresponding to different local minima in a landscape of possibilities. This could obtain in several different ways such as the local bubble location in a string landscape or in unitary quantum physics the wave function does not collapse and all possibilities are actualized.
 Max Tegmark, “The Multiverse Hierarchy,” http://arxiv.org/pdf/0905.1283v1.pdf (accessed March 6, 2012), 1.