What if …?
The shortest definition of SF I know of is that it's any form of fiction that asks “what if?”. That's SF as in speculative fiction, not the more restrictive “science fiction” or “sci-fi”.
The ISFDB policy seems like a good start, though I wouldn't be too strict with borderline cases such as technothrillers. (And of course this site isn't limited to written works.)
I don't see any mileage in refusing certain parts of SF such as alternate history or fantasy. This is too likely to generate fight upon fight about where the boundaries are. And there isn't really a science fiction community the way there is an sf community.
Be careful when using the terms “science-fiction” or “sci-fi” or “sf”, because people have different definitions. Science-fiction is usually about what might be, alternate history about what might have been, and fantasy about what can't be, but people disagree strongly on where to draw the line (is Star Wars science-fiction? Is anything with faster-than-light travel science-fiction? Is technology so advanced that it's indistinguishable from magic science-fiction?)
I especially warn against “sci-fi”, because many people who define themselves as “sf” aficionados use it in a somewhat disparaging way, limiting it to B-grade science-fiction.