Dictionary: (sci-ence fic-tion) (abbr.: SF or Sci Fi) noun: fiction based on imagined or future technological advances and major social or environmental changes, frequently portraying space or time travel and life on other planets.
Wikipedia: A genre of fiction dealing with the impact of imagined innovations in science or technology, often in a futuristic setting. Exploring the consequences of such innovations is the traditional purpose of science fiction, making it a "literature of ideas."Rod Serling's definition of fantasy and science fiction: "Fantasy is the impossible made probable. Science Fiction is the improbable made possible."
Wikipedia: Branch of Social Science Fiction/Soft Sci Fi (abbr.: spec-fic or specfic)-- an umbrella term encompassing the more fantastical fiction genres, specifically science fiction, fantasy, horror, supernatural fiction, superhero fiction, utopian and dystopian fiction, apocalyptic and post-apocalyptic fiction and alternative history in literature, as well as related static, motion, and virtual arts.
The term has been used to express dissatisfaction with what some people consider the limitations of science fiction, or otherwise to designate fiction that falls under readily stereotypical genres so that it can be pigeonholed within such categorical limits as "fantasy" or "mystery".