"Science fiction" is difficult to define, as it includes a wide range of subgenres and themes. James Blish wrote: "Wells used the term originally to cover what we would today call ‘hard’ science fiction, in which a conscientious attempt to be faithful to already known facts (as of the date of writing) was the substrate on which the story was to be built, and if the story was also to contain a miracle, it ought at least not to contain a whole arsenal of them."
Isaac Asimov said: "Science fiction can be defined as that branch of literature which deals with the reaction of human beings to changes in science and technology." According to Robert A. Heinlein, "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."
Lester del Rey wrote, "Even the devoted aficionado or fan—has a hard time trying to explain what science fiction is", and that the reason for there not being a "full satisfactory definition" is that "there are no easily delineated limits to science fiction." Author and editor Damon Knight summed up the difficulty, saying "science fiction is what we point to when we say it", while author Mark C. Glassy argues that the definition of science fiction is like the definition of pornography: you do not know what it is, but you know it when you see it.
Forrest J Ackerman is credited with first using the term "Sci-Fi" (analogous to the then-trendy "hi-fi") in 1954. As science fiction entered popular culture, writers and fans active in the field came to associate the term with low-budget, low-tech "B-movies" and with low-quality pulp science fiction. By the 1970s, critics within the field such as Knight and Terry Carr were using sci-fi to distinguish hack-work from serious science fiction. Peter Nicholls writes that "SF" (or "sf") is "the preferred abbreviation within the community of sf writers and readers". Robert Heinlein found even "science fiction" insufficient and suggested the term speculative fiction to be used instead, which has continued to be applied to "serious" or "thoughtful" science fiction.