From The Book of THoTH (Leaves of Wisdom)
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.