Science Fiction is literature about the future, telling stories of the marvels we hope to see--or for our descendants to see--tomorrow, in the next century, or in the limitless duration of time.
-- Introduction, Dream's Edge, Sierre Club Books, San Fransisco, 1980
The best definition of science fiction is that it consists of stories in which one or more definitely scientific notion or theory or actual discovery is extrapolated, played with, embroided on, in a non-logical, or fictional sense, and thus carried beyond the realm of the immediately possible in an effort to see how much fun the author and reader can have exploring the imaginary outer reaches of a given idea's potentialities.
A science fiction story is one which presupposes a technology, or an effect of technology, or a disturbance in the natural order, such as humanity, upto the time of writing, has not in actual fact experienced.
-- Best Science Fiction Stories (London, 1955)
L. Sprague De Camp
Therefore, no matter how the world makes out in the next few centuries, a large class of readers at least will not be too surprised at anything. They will have been through it all before in fictional form, and will not be too paralyzed with astonishment to try to cope with contingencies as they arise.
Lester Del Rey
... science fiction "is the myth-making principle of human nature today."
Gordon R. Dickson
In short, the straw of a manufactured realism with which the sf writer makes his particular literary bricks must be entirely convincing to the reader in it own right, or the whole story will lose its power to convince.
H. Bruce Franklin
We talk a lot about science fiction as extrapolation, but in fact most science fiction does not extrapolate seriously. Instead it takes a willful, often whimsical, leap into a world spun out of the fantasy of the author....
In fact, one good working definition of science fiction may be the literature which, growing with science and technology, evaluates it and relates it meaningfully to the rest of human existence.
Science fiction frequently tries to imagine what life would be like on a plane as far above us as we are above savagery; its setting is often of a kind that appears to us technologically miraculous. It is thus a mode of romance with a strong tendency to myth.
Vincent H. Gaddis
Science fiction expresses the dreams that, varied and modified, later becomes the visions and then the realities in scientific progress. Unlike fantasy they present probabilities in their basic structure and create a reservoir of imaginative thought that sometimes can inspire more practical thinking.
By "scientification,"... I mean the Jules Verne, H. G. Wells, and Edgar Allan Poe type of story---a charming romance intermingled with scientific fact and prophetic vision.
Science Fiction is that class of fiction which contains the currents of change in science and society. It concerns itself with the critique, extension, revision, and conspiracy of revolution, all directed against static scientific paradigms. Its goal is to prompt a paradigm shift to a new view that will be more responsive and true to nature.
-- The Cosmic Dancers (New York, 1983)
James E. Gunn
Science Fiction is the branch of literature that deals with the effects of change on people in the real world as it can be projected into the past, the future, or to distant places. It often concerns itself with scientific or technological change, and it usually involves matters whose importance is greater than the individual or the community; often civilization or the race itself is in danger.
-- Introduction, The Road To Science Fiction, Vol 1, NEL, New York 1977
Science fiction in the hand of character-draughtsman can create a new contemporary tension-of-choice, new moral decisions, and so indicate how they may be faced or flunked.
In its [science fiction's]aim it is bound, by its extrapolation of science and its use of dramatic plot, to view man and his machines and his environment as a three-fold whole, the machine being the hyphen. It also views man's psyche, man's physique and the entire life process as also a threefold interacting unit. Science fiction is the prophetic ... the apocalyptic litterature of our particular culminating epoch of crisis.
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.
To make this definition cover all science fiction (instead of "almost all") it is necessary only to strike out the word "future."
-- from: Science Fiction: its nature, faults and virtues, in The Science Fiction Novel, Advent, Chicago:1969
Science Fiction is speculative fiction in which the author takes as his first postulate the real world as we know it, including all established facts and natural laws. The result can be extremely fantastic in content, but it is not fantasy; it is legitimate--and often very tightly reasoned--speculation about the possibilities of the real world. This category excludes rocket ships that make U-turns, serpent men of Neptune that lust after human maidens, and stories by authors who flunked their Boy Scout merit badge tests in descriptive astronomy.
-- from: Ray Guns And Spaceships, in Expanded Universe, Ace, 1981