Science fantasy is a mixed genre within the umbrella of speculative fiction which combines tropes and elements from both science fiction and fantasy, and sometimes also incorporates elements of horror. The Encyclopedia of Science Fiction points out that as a genre, science fantasy "has never been clearly defined," and was most commonly used in the period 1950-1966.
Science fantasy versus science fiction
Distinguishing between science fiction and fantasy, Rod Serling claimed that the former was "the improbable made possible" while the latter was "the impossible made probable". As a combination of the two, science fantasy gives a scientific veneer of realism to things that simply could not happen in the real world under any circumstances. Where science fiction does not permit the existence of fantasy or supernatural elements, science fantasy explicitly relies upon them.
The label first came into wide use after many science fantasy stories were published in the pulp magazines, such as Robert A. Heinlein's Magic, Inc., L. Ron Hubbard's Slaves of Sleep, and Fletcher Pratt and L. Sprague de Camp's Harold Shea series. All were relatively rationalistic stories published in John W. Campbell, Jr.'s Unknown magazine. These were a deliberate attempt to apply the techniques and attitudes of science fiction to traditional fantasy subjects. The Magazine of Fantasy and Science Fiction published, among other things, all but the last of the Operation series, by Poul Anderson.
Henry Kuttner and C. L. Moore published novels in Startling Stories, alone and together, which were far more romantic. These were closely related to the work that they and others were doing for outlets like Weird Tales, such as Moore's Northwest Smith stories.
Ace Books published a number of books as science fantasy during the 1950s and '60s.
Subgenres of science fantasy
Dying Earth and Post-apocalypsis
Jack Vance's Dying Earth stories are sometimes classed as science fantasy because the cosmology used is not compatible with that conventionally accepted by science fiction. Other stories in the Dying Earth subgenre such as M. John Harrison's Viriconium novels or Gene Wolfe's Book of the New Sun are usually classed as science fantasy.
The planetary romance, a story set primarily or wholly on a single planet and illustrating its scenery, native peoples (if any) and cultures, offers considerable scope for science fantasy, in the sense of fantasy rationalized by reference to science-fictional conventions.
The works of Edgar Rice Burroughs and E. R. Eddison are probably the earliest examples of this genre, especially the John Carter of Mars series. David Lindsay's A Voyage to Arcturus, published in 1920 is one of the earliest examples of the type, although it differs from most of them in not assuming a science-fictional background of interplanetary or interstellar travel; it is rather a philosophical romance, which uses an alien planet as a background for exploring philosophical themes. C. S. Lewis' Out of the Silent Planet (1938) is an example of the same type of story, though in its case the preoccupations are theological. In both cases the magical elements are barely rationalized, and in Lewis' case stand in stark contrast to the pseudo-scientific machinery that frames the story.
Some examples of this type of science fantasy deliberately blur the already vague distinction between science fictional paranormal powers and magic; for instance, Poul Anderson's The Queen of Air and Darkness, in which aliens use psionic powers of illusion to imitate earthly myths of fairies—who are themselves traditionally regarded as magical illusionists.
In Andre Norton's Witch World series, the fantasy world is excused as a parallel universe. There are a few science fictional elements in the earlier stories of this series, which are absent from the later novels.
Sword and planet
Many works by Edgar Rice Burroughs fall into this category, as well as those of his imitators such as Otis Adelbert Kline, Kenneth Bulmer, Lin Carter, and John Norman. They are largely classed as "science fantasy" because of the presence of swords and, usually, an archaic aristocratic social system; Burroughs' own novels are, however, skeptical in spirit and almost free of any non-rationalized "fantastic" element (other than the never-explained mechanism by which John Carter gets to Mars). The graphic novel Camelot 3000 is another good example of this. The movie Krull also falls in this category, since that the movie depicts a story where a near omnipotent alien creature invades a fantasy world and the protagonists must find a way to fight back against the alien.
Science fantasy is also a popular subject for role-playing games, of both tabletop and video game varieties, the latter most popularly emphasized by games such as Final Fantasy VII and VIII, which incorporate fantasy elements such as magic in dystopian, futuristic world settings. One of the earliest pen-and-paper games to reference the term was Gamma World published by TSR in 1980 (the same company that originated Dungeons and Dragons). Other pen-and-paper RPGs typically classified as science fantasy include Rifts and Shadowrun.
The science fantasy genre has achieved moderate popularity in anime and manga. The genre was first popularized within said medium by Dragon Ball, with science fictional elements like aliens, robots and high technology coexisting with supernatural concepts like gods, demons, and powers based on the manipulation of ki, and can now be seen in more recent works such as Code Geass and Hellsing.