Over at the SF Signal blog they have a cool feature called "Mind Meld" where they ask a bunch different SF-type people a question and post the answer. This week's post is the second half of a two-parter on the definition of SF. I've made the following contribution, which expands a little on my prior post on the topic:
Defining SF has always been problematic, since there's no definitive plot element (as in Romance, or Mystery) or mood (as in Horror, Comedy, and Suspense) or setting (all Historical Novels) that helps us in defining most other genres. To this is added SF's Siamese-twin, Fantasy, a separate genre that carries the same lack of definitive plot elements, mood, or setting.
In most popular discourse the definition of SF and Fantasy ends up being definition via trope. (It has a starship, it goes here. A dragon, over there. What? Pern? Head'splode.) The other popular method of separating the twins is by splitting things up along a possible/impossible axis. This has its own consequences, such as slotting time travel and FTL into the fantasy realm. It also excludes from the SF fold stories whose point is to logically extrapolate from some fiddling with natural law-- I mean a novel about a box that adjusts the gravitational constant should be shelved in fantasy?
My own definition of SF comes in two distinct parts. First, while the story's world is not the same as the author's, there is some explicit or implicit congruency between the story's universe and the author's. There is a map from here to there, even if it's to go back to 1908 Siberia and take a hard right at Tunguska.
Second, and more important, the story is written from the perspective that the universe runs by predictable and knowable laws, and those laws are the same as those in the author's universe. If they differ at all, the difference must be explained in such a way that the story doesn't loose its connection to the author's universe.