I was having this argument earlier -- to me, the difference between 'fantasy' and 'science fiction' is an element of the supernatural. Agreed? So the counter-argument is that how can we know that anything is supernatural? Maybe it's just as yet unknown. So my counterargument to that was, "fantasy is anything that violates science both as we know it today, or anything that may violate predictably known science or something to that effect", but I was already faltering, because the whole thing about science is that we don't know what we don't know, otherwise we'd know it. So he said, "okay, so anything with faster-than-light travel or time travel is fantasy and not science fiction, even if they don't violate any other known scientific laws".
To which, I of course, had no argument, so settled with, 'yo mama was science fiction...until I did her last night, then she was my fantasy'.*
So...anyone want to throw out a definition that will help either side of the argument?
*Not really, I didn't think of that until just now.