What is science fiction?

This comes - rather laterally - out of a conversation I was having with James Smythe. What is "science fiction"?

There's a practical definition ("Whatever that individual bookseller puts in his or her Science Fiction section"), numerous academic ones, an evasive one ("Genres don't exist!") and a sort of commonly-held gut feel ("I'll know it when I see it. Also, rockets.")

My own background is in marketing, so let's approach it that way: if "Science Fiction" was a product that we wanted to s...