What is Science Fiction?

ParadoXIII asks: "I would like to know what the /. community defines as science fiction. What separates it from fantasy or from regular stories? Where and when did the genre originate? What are the goals and purposes of its writers (if any)?" We've probably gone over this in several scattered articles from Star Wars to The Matrix, but I think it still deserves a dedicated discussion. In short, I define Science Fiction as: "fiction in which the characters react to fictional developments of science". But maybe you all think that's too generic. Thoughts?