In 1972 Darko Suvin defined sf [sic] as “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”. By “cognition” Suvin appears to mean the seeking of rational understanding, and by “estrangement” something akin to Bertolt Brecht's Verfremdungseffekt, defined in 1948 thus: “A representation which estranges is one which allows us to recognize its subject, but at the same time make it seem unfamiliar.” Perhaps the most important part of Suvin's definition, and the easiest with which to agree, is the emphasis he puts on what he and others have called a novum, a new thing — some difference between the world of the fiction and what Suvin calls the “empirical environment”, the real world outside. The presence of a novum is insufficient in itself, of course, to define sf, since the different and older tradition of fantasy likewise depends on the novum.

— Brian M. Stableford, John Clute and Peter Nicholls, “Definitions of SF”, The Encyclopedia of Science Fiction, eds John Clute and David Langford (4th edn, 2021).