Sentence:  The first credible suggestion that <WSD>alternate universes</WSD> might exist came in the early 1950s when a young physics graduate student named Hugh Everett was toying with some of the more bizarre implications of quantum mechanics. 
Meaning of the word marked with <WSD> tags:  A hypothetical world situated in a different dimension of space and time than the real world consisting of the universe known and experienced by human beings.