There are almost as many definitions of science fiction and fantasy (SF & F) as there are readers and writers — so many that we're often reduced to throwing up our hands and saying, "It's what I mean when I point to it." But surely we can get closer to a definition than that. How about this:
SF stories are those that could not happen without some element of science, or some imagined change (futuristic or otherwise) from the world as we know it today. Fantasy also takes place in otherworldly settings, but in this case, the worlds are usually magical or mythical.
SF stories tend to be based on, well — science, or worlds that
seem possible or plausible, based on what we know or can guess about science.
Fantasy is a joining of myth with imagination. If science fiction is drawn from our rational side, fantasy wells up from the subconscious.