The canon of post-war science fiction. John Wyndham’s and Arthur Clarke’s novels.

Science fiction is a broad genre of fiction that often takes place in the future and usually involves speculations based on current science. It is commonly abbreviated as SF or sci-fi. Science fiction is found in books, art, television, movies, games, theater, and other media.

In organizational or marketing contexts, science fiction can be synonymous with the broader definition of speculative fiction, encompassing creative works incorporating imaginative elements not found in contemporary reality; this includes fantasy, horror, and related genres.

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 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." 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 column 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.

Science fiction often involves one or more of the following elements:

• A setting in the future or on an alternative time line.

• A setting in outer space or involving aliens or unknown civilizations.

• The discovery or application of new scientific principles, such as time travel or psionics (the practice of using a variety of psychic abilities), or new technology, such as nanotechnology, faster-than-light travel or robots.

• Political or social systems different from those of the known present or past.

Exploring the consequences of such differences is the traditional purpose of science fiction, making it a "literature of ideas".