In the 1950s Judith Merril (1923-1997) advocated the use of this term as consistent with her sense that Science Fiction could be understood as an aspect of Fantasy as a whole. In Critical Terms for Science Fiction and Fantasy: A Glossary and Guide to Scholarship (1986), Gary K Wolfe (1946- ) suggests that the term, defined as tightly as possible, "refers to a genre in which devices of fantasy are employed in a 'science-fictional' context (related to but distanced from the 'real world' by time, space, or dimension)".
Both Heroic Fantasy and Rationalized Fantasy texts have often been called SF, and both frequently underpin their worlds through explanations which draw on history (as in Robert E Howard's Conan series) or science (as in the elaborate explanations of how Magic works in Poul Anderson's Operation Chaos [coll of linked stories 1971]). In the works of authors like Leigh Brackett, Andre Norton and Marion Zimmer Bradley – and many others – Planetary-Romance venues can tend to take on Fantasyland airs very swiftly.
Perhaps the most useful application of the term is to Dying-Earth tales – like those by Jack Vance, Michael Moorcock and Gene Wolfe – in which fantasy-like tales are told in venues not necessarily very well understood by the characters involved. It is here that SF and fantasy as a whole are closest: to invoke historical or scientific explanations for happenings that seem magical is likely to arouse a sense of Time Abyss, because these explanations will almost certainly refer back to wisdom or science that has been forgotten for aeons. In Dying-Earth tales history and science are like the stories which underlie most fantasy narratives, and when protagonists in such tales discover what makes their worlds tick, the effect is less that of conceptual breakthrough than of Recognition. [JC]
see also: Technofantasy.