Ask any writer or fan to define science fiction (or fantasy) — and lord knows, people have asked — you’ll get a lot of contradictory and only sometimes helpful answers. Here are just some that have been sitting around in my saved links for awhile:
“Science fiction writers don’t admit magic, they don’t admit UFOs even, but they accept as given these two magical properties [time travel and travel that exceeds the speed of light, so that, in a sense, even their science fiction is built on fantasy.” Robert J. Sawyer
“Magic gets to break the laws of nature. Science doesn’t. And that goes for science-fiction, too: It might do things that aren’t currently possible, but as soon as it starts breaking the laws of physics it has stepped out of science and into fantasy.” – Mary Robinette Kowal
“The failure mode of science fiction is NOT ‘fantasy,’ it is ‘bad science fiction.'” – John Scalzi
“…the term ‘science fiction’ is a misnomer, that trying to get two enthusiasts to agree on a definition of it leads only to bloody knuckles; that better labels have been devised (Heinlein’s suggestion, ‘speculative fiction,’ is the best, I think), but that we’re stuck with this one; and that it will do us no particular harm if we remember that, like ‘The Saturday Evening Post,’ it means what we point to when we say it.” – Damon Knight
“What can not be defined are genres. You can’t define poetry, the novel, tragedy, pornography, comics… and you cannot define academic criticism either. Now, you can describe all of them in perfectly useful ways. You can describe them so people can recognize them. So that particular provisional job that you have to do can get done. You just can not specify their necessary and sufficient conditions. So there are no root examples, no borderline cases. With different descriptions, different borderlines come into being. I think the genre or at least genre criticism, might take a major step forward if it simply threw out the term ‘definition.’ The term ‘description,’ would do perfectly well. The point is, if someone asks you, ‘How would you “describe” science fiction,’ the proper answer is: ‘For what particular purpose do you want this description?’ This is not the mood in which the question ‘How do you define sf?’ gets asked.” – Samuel R. Delaney
As I’ve noted before, I usually find strict definitions of fantasy and science fiction (or any genre) pretty limiting. I think I most like what Delaney says above.