Science fiction is a genre of fiction dealing with imaginary but more or less plausible (or at least nonsupernatural) content such as future settings, futuristic science and technology, space travel and aliens. Exploring the consequences of scientific innovations is one purpose of science fiction, making it a "literature of ideas".

Science fiction is largely based on writing rationally about alternative possibilities. It is similar to but differs from fantasy in that, within the context of the story, its imaginary elements are largely possible within scientifically established or scientifically postulated laws of nature (though some elements in a story might still be pure imaginative speculation).

The settings for science fiction are often contrary to known reality, but most science fiction relies on a considerable degree of suspension of disbelief, which is facilitated in the reader's mind by potential scientific explanations or solutions to various fictional elements. Science fiction criteria include:

A time setting in the future, in alternative timelines, or in a historical past that contradicts known facts of history or the archaeological record.

A spatial setting or scenes in outer space (e.g., spaceship travel), on other worlds, or on subterranean earth.

Characters that include aliens, mutants, androids, or humanoid robots.

Technology that is futuristic (e.g., ray guns, teleportation machines, humanoid computers).

Scientific principles that are new or that contradict known laws of nature--for example, time travel, wormholes, or faster-than-light travel.

New and different political or social systems (e.g., a dystopia, or a post-apocalyptic situation where organized society has collapsed).