Science Fiction is that class of prose narrative treating of a situation that could not arise in the world we know, but which is hypothesised on the basis of some innovation in science or technology, or pseudo-science or pseudo-technology, whether human or extra-terrestrial in origin.— Kingsley Amis
… science fiction presents with verisimilitude the human effects of spectacular changes in our environment, changes either deliberately willed or involuntarily suffered.
The business about Xi space and its effects in our universe occupies the position given in ordinary fiction to matters of human situation or character; in this sense it is the hero of the story. Ideas as hero is the basis of a great deal of science fiction, corresponding to what Edmund Crispin in another of his incarnations has called the plot as hero type of detective story, that traditional category in which the circumstances of the crime determine the process of its explanation and thus furnish the entire structure of the narrative. The primacy of idea means that a good science fiction story of this kind will sound good in paraphrase, and in this direction lies some support to the plea that stylistic adequacy is all one need demand from examples of the idea-category, which is not a vehicle for the verbal imagination. I might broaden the notion of idea as hero by pointing out that an idea of scientific interest, or even of scientific respectability, is no requirement, provided as always that conceivability is not outraged.— Kingsley Amis
I like to collect definitions of SF.