Science fiction is a genre of fiction in which at least part of the narrative depends on the impact of science, either real or imagined, to generate settings or events which have not yet occurred in reality (and may never do so).

　　Robert A. Heinlein, a leading writer of science fiction, wrote "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." He immediately adds that if you "strike out the word 'future' it can apply to all and not just almost all SF."

　　SF author Theodore Sturgeon wrote "A good science fiction story is a story about human beings, with a human problem, and a human solution, that would not have happened at all without its science content." .

　　Of course, both of these authors are defining what they consider to be good science fiction. Not all writers or fans agree on how important realism and characterization are in science fiction. Any story, film, game, or toy that includes aliens, spaceships, time travel, or the future is called science fiction.

　　Definition and scope

　　The borders of the genre are difficult to define, and the dividing lines between its subgenres are often fluid. In his book of essays, Strong Opinions, Vladimir Nabokov half-seriously argues that if we were rigorous with our definitions, Shakespeare's play The Tempest would have to be termed science fiction.

　　Broadly speaking, the science fiction genre is concerned with the effects of science or technology on society or individuals. These effects may be epic in scope or personal. The science-fictional elements may be imagined or rooted in reality, original or clich. See science fiction genres for a list of some genres.

　　Science fiction and fantasy

　　A science fiction story may be firmly rooted in real scientific possibilities (see Hard science fiction) as they are understood at the time of writing, as in Arthur C. Clarke's novel A Fall of Moondust, or highly imaginative, set in an extraterrestrial civilization or a parallel universe, as in Isaac Asimov's novel The Gods Themselves.

　　Some science fiction portrays events that fall outside of science as currently understood, as in Ray Bradbury's The Martian Chronicles. But one alternate viewpoint on such tales is to view them not from the current era's understanding of science, but to view the tale in the context of the known science during the time the tale was written. Another example of that would be Heinlein's The Man Who Sold the Moon, which postulated a private enterprise exploration of the earth's moon decades in advance of the real events in 1969— thus a contemporary reader might instead take the work as a member of the subgenre Alternate history, rather than the Hard science fiction work it was at the time of its publication.

　　Also, different readers have different ideas about what counts as scientifically "realistic"; an uneducated person will have different expectations about what science can do than a professional physicist. As Clarke himself stated, "any sufficiently advanced technology is indistinguishable from magic" (third in Clarke's three laws). Thus, even fiction that depicts innovations ruled out by current scientific theory, such as stories about faster-than-light travel, may still be classified as science fiction, as they are in the popular Honorverse novels and stories by David Weber.

　　Accordingly, the borderline between fantasy and science fiction is blurred, and many bookstores shelve science fiction and fantasy together. There is a substantial overlap between the audiences of science fiction and fantasy literature, and many science fiction authors have also written works of fantasy. Fans often nominate works of fantasy for SF awards such as the Hugo and Nebula, clearly indicating a substantial overlap among readers.

Indeed, it can be argued that science fiction is simply a modern form of fantasy. According to this view, the elements that would previously have been presented as fantasy (e.g., magic, shapeshifting, divination, mind-reading, fabulous beasts, and so on) are rationalized or supported through scientific or explanations such as marvelous devices, mutation, psychic abilities, aliens, etc.

　　This definition is resisted by some scholars and writers who attempt to define the genre's aspects more sharply, and advocate an aspiration to present a world without mystical or supernatural forces. For example, in such works as Metamorphoses of Science Fiction, Darko Suvin emphasises a cognitive element in SF. According to Suvin, the purpose of science fiction is to introduce scientific or technological novelties in order to create narratives that enable us to perceive everyday reality at a reflective distance. He uses the term cognitive estrangement to label this effect.

　　Some SF clearly exhibits this aspiration, but not all. As a result, some theorists are able to emphasise the difference between SF and fantasy, while others emphasise continuity. It is also common to see narratives described as being essentially SF but "with fantasy elements." More recently, the term "science fantasy" has been increasingly used to describe such material.

　　Science fiction and mainstream literature

　　Science fiction can overlap with more mainstream fiction.

　　If the society, the person, the technology, and the scientific knowledge base in the story are all drawn from observed reality, without much detail about the scientific aspects, the story may be classed as mainstream, contemporary fiction rather than as science fiction, like Marooned by Martin Caidin, or virtually all the novels by Tom Clancy. If the characters' thoughts and feelings about the laws of the universe, time, reality, and human invention are unusual and tend toward existential re-interpretation of life's meaning in relation to the technological world, then it may be classed a modernist work of literature that overlaps with the themes of science fiction. Examples include Thomas Pynchon's Gravity's Rainbow, William Burroughs's Nova Express, Kazuo Ishiguro's Never Let Me Go, and much of the work of Kurt Vonnegut, Philip K. Dick, and Stanis?aw Lem.

　　. Purpose of science fiction

　　Science fiction has often been concerned with the great hopes people place in science but also with their fears concerning the negative side of technological development; the latter is expressed in the classic theme of the hubristic scientist who is destroyed by his own creation.

　　Much science fiction attempts to generate a sense of wonder, or awe, from the setting, circumstances, or ideas presented. Paradigm shifts may be used to induce a sense of shock, or a change in the frame of reference for the reader.

　　A popular misconception is that science fiction attempts to predict the future. Some commentators may even go so far as to judge the "success" of a work of science fiction on the accuracy of its predictions. However, while some science fiction is set in the future, most authors are not attempting literally to predict it; instead, they use the future as an open framework for their themes. As Ray Bradbury put it, "People ask me to predict the future, when all I want to do is prevent it. "A science fiction writer is generally not trying to write a history of the future that they believe will happen, any more than a writer of westerns is trying to create a historically accurate depiction of the old West. Writers are as likely to write of a future that they hope will not happen as they are to write about a future they think will happen. Future societies and remarkable technological innovations are presented as enabling devices for cognitive exploration - or simply for entertainment - and the narratives are not meant to be predictive in any simple way. There are exceptions, however, especially in early science fiction.

According to Eric S. Rabkin:

　　"The touchstone for scientific fiction, then, is that it describes an imaginary invention or discovery in the natural sciences. The most serious pieces of this fiction arise from speculation about what may happen if science makes an extraordinary discovery. The romance is an attempt to anticipate this discovery and its impact upon society, and to foresee how mankind may adjust to the new condition."