Science fiction (sometimes shortened to sf or sci-fi) is a genre of speculative fiction, which typically deals with imaginative and futuristic concepts such as advanced science and technology, space exploration, time travel, parallel universes, and extraterrestrial life. Science fiction can trace its roots to ancient mythology. It is related to fantasy, horror, and superhero fiction and contains many subgenres. Its exact definition has long been disputed among authors, critics, scholars, and readers. Science fiction, in literature, film, television, and other media, has become popular and influential over much of the world. It has been called the "literature of ideas", and often explores the potential consequences of scientific, social, and technological innovations. It sometimes serves as an outlet to facilitate future scientific and technological innovations. Besides providing entertainment, it can also criticize present-day society and explore alternatives. It is also often said to inspire a "sense of wonder". Definitions American science fiction author and editor Lester del Rey wrote, "Even the devoted aficionado or fan—has a hard time trying to explain what science fiction is," and the lack of a "full satisfactory definition" is because "there are no easily delineated limits to science fiction." According to Isaac Asimov, "Science fiction can be defined as that branch of literature which deals with the reaction of human beings to changes in science and technology." Robert A. Heinlein wrote that "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." Part of the reason that it is so difficult to pin down an agreed definition of science fiction is because there is a tendency among science fiction enthusiasts to act as their own arbiter in deciding what exactly constitutes science fiction. Damon Knight summed up the difficulty, saying "science fiction is what we point to when we say it." David Seed says it may be more useful to talk around science fiction as the intersection of other, more concrete, genres and subgenres. Alternative terms Forrest J Ackerman has been credited with first using the term "sci-fi" (analogous to the then-trendy "hi-fi") in about 1954; the first known use in print was a description of Donovan's Brain by movie critic Jesse Zunser in January 1954. As science fiction entered popular culture, writers