There is no commonly accepted definition of Science Fiction - all of the above have been proposed. I'm going to stick with the third and fourth, but the other two aren't as silly as they seem. These are taken from a book called "Turning Points: Essays on the Art of Science Fiction" edited by Damon Knight (1977). Number 1 is attributed to Norman Spinrad, number 2 is an obvious generalization of number 1 (and I'm sure I've seen it used somewhere), number 3 is attributed to Theodore Sturgeon, and number 4 is Reginald Bretnor's definition, paraphrased by Robert Heinlein. "Extrapolation" and "future" are two words also strongly associated with SF.