A bunch of SF writers are asked to define SF; hilarity ensues.
My favorite two definitions that I recall are: "SF is whatever I point at when I say SF", and "SF is when you push a button to make something impossible happen, while fantasy is when you wave your have to make something impossible happen."
I think that Damon Knight said the former, but I don't remember where I got the latter, so if anyone knows, feel free to give my memory a jog. Interestingly, that second definition still fails to classify Star Wars! Personally I consider Star Wars to be SF, although it's so strongly shot through with fantastic tropes that maybe it's best thought of as fantasy set in space instead of Middle Earth. But for some reason, that answer just doesn't feel right to me.