Definition of Science Fiction (Score 1)

"The major distinction between fantasy and science fiction is, simply, that science fiction uses one, or a very, very few new postulates, and develops the rigidly consistent logical consequences of these limited postulates. Fantasy makes its rules as it goes along...The basic nature of fantasy is "The only rule is, make up a new rule any time you need one!" The basic rule of science fiction is "Set up a basic proposition--then develop its consistent, logical consequences."

-- John Campbell, 1966

"'Hard' science fiction ... probes alternative possible futures by means of reasoned extrapolations in much the same way that good historical fiction reconstructs the probable past. Even far-out fantasy can present a significant test of human values exposed to a new environment. Deriving its most cogent ideas from the tension between permanence and change, science fiction combines the diversions of novelty with its pertinent kind of realism."

-- Jack Williamson

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."

Robert A. Heinlein, 1981

Quotes from http://www.panix.com/~gokce/sf_defn.html