Science fiction is difficult to define as it includes a wide range of subgenres and themes. Author and editor Damon Knight summed up the difficulty by stating that "science fiction is what we point to when we say it". Vladimir Nabokov argued that were we rigorous with our definitions, Shakespeare's play The Tempest would have to be termed science fiction.

According to SF writer Robert A. Heinlein, "a handy short definition of almost all science fiction might read: realistic speculation about possible future events, based solidly on adequate knowledge of the real world, past and present, and on a thorough understanding of the nature and significance of the scientific method." Rod Serling's stated definition is "fantasy is the impossible made probable. Science Fiction is the improbable made possible."

Forrest J. Ackerman publicly used the term "sci-fi" at UCLA in 1954, though Robert A. Heinlein had used it in private correspondence six years earlier. As science fiction entered popular culture, writers and fans active in the field came to associate the term with low-budget, low-tech "B-movies" and with low-quality pulp fiction science fiction. By the 1970s, critics within the field such as Terry Carr and Damon Knight were using "sci-fi" to distinguish hack-work from serious science fiction, and around 1978, Susan Wood and others introduced the pronunciation "skiffy." Peter Nicholls writes that "SF" (or "sf") is "the preferred abbreviation within the community of sf writers and readers." David Langford's monthly fanzine Ansible includes a regular section "As Others See Us" which offers numerous examples of "sci-fi" being used in a pejorative sense by people outside the genre.

Source: http://en.wikipedia.org/wiki/Science_fiction