Science fiction (abbreviated SF or sci-fi with varying punctuation and case) is a broad genre of fiction that often involves speculations based on current or future science or technology. Science fiction is found in books, art, television, films, 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 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).

Science fiction is largely based on writing entertainingly and rationally about alternate possibilities in settings that are contrary to known reality. These include:

• A setting in the future, in alternative time lines, or in a historical past that contradicts known facts of history or the archeological record
• A setting in outer space, on other worlds, or involving aliens
• Stories that contradict known or supposed laws of nature
• Stories that involve discovery or application of new scientific principles, such as time travel or psionics, or new technology, such as nanotechnology, faster-than-light travel or robots, or of new and different political or social systems

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