We call ourselves the Glasgow Science Fiction Writers’ Circle, but don’t get hung up on the “science fiction” part. We usually shorten it to SF and allow it to stand for any sort of fiction with fantastical or speculative content. Which means that we have critiqued stories that could be described as science fiction, epic fantasy, horror, supernatural, urban fantasy, magic realism, slipstream, space opera, cyberpunk, steampunk, new weird, alternative history, and a whole bunch of stuff not covered by any of those categories. A speculative element isn’t mandatory either in pieces we critique. And neither does it have to be fiction.
SF means whatever you want it to mean.
But if you want to get all technical and literary about it, Hal Duncan has prepared some thoughts.