Coincidentally, I recently got into an interesting argument with my father. He'd just seen Captain Marvel and got defensive when my friend referred to it as a superhero movie; he insisted that it was a science fiction movie.
I probably shouldn't enjoy being empirically correct so much, but this dispute gave me the opportunity to demonstrate that he was, quite simply, wrong.
Disregarding the movie, it's the attempt to identify sci-fi that is interesting. In the same way the judge couldn't define pornography "but knows it when I see it", there is not one standing definition. However, the greats who established the genre agreed on some important basics which hold water today. Cribbed from Wikipedia:
Heinlein: "...realistic speculation about possible future events, based solidly on adequate knowledge of the real world, past and present, and on a thorough understanding of the nature and significance of the scientific method."
Wells: "...a conscientious attempt to be faithful to already known facts (as of the date of writing) was the substrate on which the story was to be built, and if the story was also to contain a miracle, it ought at least not to contain a whole arsenal of them."
Shippey: "...the basic building block and distinguishing feature of a science fiction novel is the presence of the novum... a discrete piece of information recognizable as not-true, but also as not-unlike-true, not-flatly- (and in the current state of knowledge) impossible."
It's Wells' point that led to me leaving this comment. Don't get me wrong - I get genuinely excited every time REDACTED shows up in a Stephenson novel... but his immortality is, for me, uncomfortably close to a miracle.
Now, sometimes this is preferable; hands up if you would prefer to not know the word Miti-chlorian.