Toward a Definition of "Science Fiction"
From Kim Stanley Robinson, "Notes for An Essay on Cecelia Holland," Foundation (UK) 40 (Summer 1987): 54-5.
Science fiction is an historical literature. In fact this historicity defines the genre. The simplest way to say this is, "Science fiction stories are set in the future." Unpack this statement and we get something like the following: "In every sf narrative, there is an explicit or implicit fictional history that connects the period depicted to our present moment." The reader assumes that, starting from out present, a sequence of events will lead us to the "present" described in the narrative.
Not all science fiction stories are set in the future. There are, for instance, the stories we call "alternative histories". But we can easily explain why the alternative history is part of sf, by revising the statement above to say, "In every sf narrative, there is an explicit or implicit fictional history that connects the period depicted to our present moment, or to some moment of our past." It is the same process, connecting to a different point in time. Because no sf story describes the actual future that will ensue in the real world, one could even say that all science fiction narratives are alternative historiesÑsome branching away from our present, others branching away from some moment of our past.
Using this historical definition we can distinguish sf from fantasy: sf is historical, fantasy is ahistorical [a = "not"]. Sometimes the worlds described in fantasies have histories, but they do not connect with our own. Fantasies are not alternative history, but alternative reality.
This definition can also explain the hybrid called "science fantasy". Typically such a narrative is set in the very far future; accepting this, we label the text sf. But we cannot truly imagine millions of years of history, so the connection is not a felt relationship, and the text seems like a fantasy narrative, about a time that never connects to ours. Calling the narrative "science fantasy" labels this clash of generic impressions.
To say that sf is historical fiction is not to say that it is the same thing as the genre called "historical fiction". The two genres are not the same, nor are they mirror images; the future is fundamentally different from the past.
But the two literary genres are more alike, in some respects, than either is like the literary mainstream. They share some methods and concerns, in that both must describe cultures that cannot be physically visited by the reader; thus both are concerned with alien cultures, and with estrangement. And both genres share a view of history which says that  times not our own are yet vitally important to us and worth writing about.
- Foundation combines UK with US conventions for format; they're also less fanatical than most US and Canadian publications about proofreading.
- Like most stories, SF stories are usually told in the past tense; hence, the "present" in an SF story, usually, is the present of the Narrator and assumed Audience, with the Narrator telling a story set in their immediate or recent or remote past.
- Some fantasies may state or imply that they're set in an Age (Epoch, Era) of the Earth long prior to our own, during the time when giants walked and unicorns roamed and the elves and dwarves were still with us--and magic worked.
Janeen Webb, "I Know Who I Am, But What's My Brand Name?" Australian Science Fiction Review, Second Series, #10, 2.5 (Sept. 1987): 13-20. Rev. article of 12 books in the Women's Press sf series.
(Webb's title alludes to a repeated line in Joanna Russ's The Female Man.)
[The aim of the Women's Press SF series is] '. . . to publish science fiction by women about women; to present exciting and provocative feminist images of the future that will offer an alternative vision of science and technology, and challenge male domination of the science fiction tradition itself'. [Publication of such works] is part of a concerted feminist challenge to patriarchal ideology in social, cultural, and political terms; the choice of sf underlines what Marilyn Hacker called 'the pertinence and necessity of speculative fiction to feminists'.
Why is sf valuable to the feminist movement?...If Rosemary Jackson is correct in claiming that sf is primarily subversive, it is easy to argue that feminist sf deliberately uses the form to undermine the usual (read patriarchal) 'dominant philosophical and epistemological orders' of fiction. Jackson claims that the fantastic subverts the 'nominal unities of time, space and character, as well as questioning the possibility, or honesty of fictional re-presentation of those unities' and that it 'moves towards a dismantling of the "real", most particularly of the concept of "character" and its ideological assumptions' (Fantasy: The Literature of Subversion 175-6). Such a method is obviously useful in feminist writing, wherein, by positing alternative 'realities', sf challenges existing cultural value, changing women from perceived to perceiver, passive to active, object to subject, and thereby attacking the negative myths that have been internalized as part of social conditioning.
There are many ways in which the feminist sf in this series sets out to subvert patriarchal assumptions in order to build a positive female self-image. They range from wish-fulfilling utopian futures that offer alternative universes where women are dominant, successful, and free...through worlds in which women simply operate as intelligent protagonists as a matter of course...to earnest lectures on the problematic relationship between the sexes...to  admonitory dystopian futures in which women's subjugation and debasement is acute....
Such fiction reinforces the feminist critical perspective that places the experience of women under patriarchy at the centre of the text. Discussion of this situation is facilitated by the conventions of sf, which invite investigation of behavioural and cultural conventions: when aliens drop in to visit, study, or just plain interfere, someone has to give etiquette lessons.* * *
The inverse of the 'educating the alien' scenario occurs when humans visit other worlds. In these cases, human (read male) values are juxtaposed with those of more enlightened societies, so that the human protagonist is educated by the alien culture. This is the traditional method of utopian fiction....
Another method of exploring the cultural repression of women is the presentation of dystopian futures derived from our own civilization....(13-14)
Darko Suvin, Metamorphoses of Science Fiction: On the Poetics and History of a Literary Genre (New Haven: Yale UP, 1979), 6-8.
Thus SF takes off from a fictional ("literary") hypothesis and develops it with totalizing ("scientific") rigor . . . . The effect of such factual reporting of fictions is one of confronting a set normative system--a Ptolemaic-type closed world picture--with a point of view or look implying a new set of norms; in literary theory this is known as the attitude of estrangement. This concept was first developed on non- naturalistic texts by the Russian Formalists . . . and most successfully underpinned by an anthropological and historical approach in the work of Bertolt Brecht, who wanted to write "plays for a scientific age." While working on a play about the prototypical scientist, Galileo, he defined this attitude ("Verfremdungseffekt") in his Short Organon for the Theatre: "A representations which estranges is one which allows us to recognize its subject, but at the same time makes is seem unfamiliar." And further: for somebody to see all normal happenings in a dubious light, "he would need to develop that detached eye with which the great Galileo observed a swinging chandelier. He was amazed by the pendulum motion as if he had not expected it and could not understand its occurring, and this enabled him to come at the rules by which it was governed." Thus, the look of estrangement is both cognitive and creative; and as Brecht goes on to say, "one cannot simply exclain that such an attitude pertains to science, but not to art. Why should not art, in its own way, try to serve the great social task of mastering Life?"  * * *
2.2. SF is then, 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 [roughly, "everyday, environment governed by natural laws"].
Estrangement differentiates Sf from the "realistic" literary mainstream extending from the eighteenth century into the twentieth. Cognition [i.e., disciplined thought] differentiates it not only from myth, but also from the folk (fairy) tale and the fantasy. The folktale also doubts the laws of the author's empirical world, but it escapes out of its horizons and into a closed collateral world indifferent to cognitive possibilities. It does not use imagination as a means of understanding the tendencies latent in reality, but as an end sufficient unto itself and cut off from the real contingencies. The stock folktale accessory, such as the flying carpet, evades the empirical law of physical gravity--as the hero evades social gravity [i.e., by rising in the class structure]--by imagining its opposite. This wish-fulfilling element is its strength and its weakness, for it never pretends that a carpet could be expected to fly--that a humble third son could be expected to become king--while there is gravity. It simply posits another world beside yours where some carpets do, magically, fly, and some paupers do, magically, become princes, and into which you cross purely by an act of faith and fancy. Anything is possible in a folktale, because a folktale is manifestly impossible. Furthermore, the lower-class genre of folktale was from the seventeenth-eighteenth centuries on transformed into the more compensatory, and often simplistic, individualist fairy tale. Therefore, SF retrogressing into fairy tale (for example, "space opera" with a hero-princess-monster triangle in astonautic costume) is committing creative suicide.(7-8)
NOTE: In Suvin's theory, would the Star Wars films and Close Encounters of the Third Kindbe SF? Would they be "SF retrogressing into fairy tale"? If the latter, would they be "bad" SF?