What is Sci-Fi?

A question was posed on twitter, as to whether sci-fi was actually a genre or simply a type of setting in which other stories adventures, romances, etc. were told? A similar question is often asked as to what the difference is, if any, between science fiction and fantasy? Are they separate entities or two facets of a single entertainment type? As we live more and more in a world in technological flux what was science fiction yesterday is general fiction now. So what is sci-fi? Is it a separate genre, a type of setting, another sort of fantasy, or something else?
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