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".[6] Vladimir Nabokov argued that were we rigorous with our definitions, Shakespeare's play The Tempest would have to be termed science fiction.[7]
According to science fiction 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."[8] Rod Serling's stated definition is "fantasy is the impossible made probable. Science Fiction is the improbable made possible."[9]
Lester Del Rey wrote, "Even the devoted aficionado– or fan- has a hard time trying to explain what science fiction is," and that the reason for there not being a "full satisfactory definition" is that "there are no easily delineated limits to science fiction." [10]
Author Mark C. Glassy stated that the definition of science fiction was very much like the definition of porn; you don't know what it is, but you know it when you see it.[11]
Forrest J. Ackerman publicly used the term "sci-fi" at UCLA in 1954,[12] though Robert A. Heinlein had used it in private correspondence six years earlier.[13] 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 science fiction.[14][15][16] 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,[17] 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."[18] 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.