Science Fiction is the extrapolation of known science (or existing technology) based on current knowledge, and the effect it has on society or an individual. Stanley Schmidt, long-time editor of the Hard SF magazine Analog, defined it like this: "My definition of science fiction is simply fiction in which some element of speculation plays such an essential and integral role that it can't be removed without making the story collapse, and in which the author has made a reasonable effort to make the speculative element as plausible as possible. Anything that doesn't meet those requirements is not science fiction at all, as far as I'm concerned, so there's no need for a separate term like "Hard SF" to distinguish it from "other" kinds of SF."
No disrespect to Stan, who did me a very nice favor a few years back, but of course
Analog is going to say that. They're the flagship of the whole John W. Campbell
/Astounding school of hard and hardish SF, which is a proud and noble tradition, to be sure, but it's not the only branch of SF that matters . ..
What about Ray Bradbury, Theodore Sturgeon, Fritz Leiber, C. L. Moore, Zenna Henderson, Richard Matheson, Philip K. Dick, Rod Serling,and the like? Not to mention magazines like Asimov's or Interzone or Omni or F&SF?
SF is a big umbrella, which covers Analog-style SF as well as many other varieties. And SF overlaps with fantasy and horror under an even bigger umbrella.
Definitions may matter in science, but in the arts . . . maybe not so much.
Says the guy who is editing lots of Weird Westerns these days, which combine SF, fantasy, horror, steampunk, and western tropes--to, hopefully, entertaining effect!
Honestly, I love stories that trash borders and blur genres and, left to my own devices, tend to gravitate toward them . ..