There have been many attempts at defining science fiction. This is a list of definitions that have been offered by authors, editors, critics and fans over the years since science fiction became a genre. Definitions of related terms such as "science fantasy", "speculative fiction", and "fabulation" are included where they are intended as definitions of aspects of science fiction or because they illuminate related definitions—see e.g. Robert Scholes's definitions of "fabulation" and "structural fabulation" below. Some definitions of sub-types of science fiction are included, too; for example see David Ketterer's definition of "philosophically-oriented science fiction". In addition, some definitions are included that define, for example, a science fiction story, rather than science fiction itself, since these also illuminate an underlying definition of science fiction.
The Encyclopedia of Science Fiction, edited by John Clute and Peter Nicholls, contains an extensive discussion of the problem of definition, under the heading "Definitions of SF". The authors regard Darko Suvin's definition as having been most useful in catalysing academic debate, though they consider disagreements to be inevitable as science fiction is not homogeneous. Suvin's cited definition, dating from 1972, is: "a literary genre whose necessary and sufficient conditions are the presence and interaction of estrangement and cognition, and whose main formal device is an imaginative framework alternative to the author's empirical environment". The authors of the Encyclopedia article—Brian Stableford, Clute, and Nicholls—explain that, by "cognition", Suvin refers to the seeking of rational understanding, while his concept of estrangement is similar to the idea of alienation developed by Bertolt Brecht, that is, a means of making the subject matter recognizable while also seeming unfamiliar.
Tom Shippey compared George Orwell's Coming Up for Air (1939) with Frederik Pohl and C. M. Kornbluth's The Space Merchants (1952), and concluded that the basic building block and distinguishing feature of a science fiction novel is the presence of the novum, a term Darko Suvin adapted from Ernst Bloch and defined as "a discrete piece of information recognizable as not-true, but also as not-unlike-true, not-flatly- (and in the current state of knowledge) impossible."
The order of the quotations is chronological; quotations without definite dates are listed last. The list below omits Hugo Gernsback's later redefining of the term "science fiction". According to anthologist, populist and historian of the genre Sam Moskowitz (1920–1997), Gernback's final words on the matter were: "Science fiction is a form of popular entertainment which contains elements of known, extrapolation of known or logical theoretical science". The list also omits John W. Campbell's infamous "Science fiction is what I say it is".