On Science Fiction and Fantasy, we go as far as banning “what is the definition of this subgenre” and “what subgenre is this movie”. Our experience is that such questions only ever get two kinds of answers, both useless:
- summaries of the definition of a subgenre in Wikipedia;
- sterile debates about the exact location of the boundaries between subgenres (as it there were precise boundaries) and about what word is appropriate for what subgenre.
I'm offering this as a data point; the genre scope of this site is broader than SF&F's, which might change the situation. Nonetheless, I do expect that most questions to be asked about genre will be non-constructive.
The case of Twilight: science fiction or fantasy? illustrates why we have this policy on SF&F. Do you want the official genre? Wikipedia says “supernatural romance fantasy”, IMDB says “adventure drama fantasy”. Do you want other possible interpretations? The one answer posted so far explains how there's a continuum and the movie is at some point on that continuum. When such topics come up, I like to argue that Star Wars is fantasy (lasers that stop? “long ago in a galaxy far away”?) and Discworld is science fiction (it's all rigorously explained by the laws of narrative imperative). That's not a consensual point of view, but it's not one you can refute, so why bother?