A Quick and Dirty Definition of Sci-fi

To me, science fiction and fantasy are the modern versions of the ancient myths, where authors use fantastic elements and scenarios to show how the world could, or should work. What separates science fiction from fantasy is that authors insist there is some rational explanation for the existence of these marvels, whether the logic can hold up in reality, or not. For instance, in science fiction, zombies exist due to a mutation or virus (like in Mira Grant's Feed series), but in fantasy, zombies exist due to a necromancer's spell. The evolution of zombies in reality is debatable, but many authors argue that it could happen, even if they aren't exactly certain how this would take place. The same thing happens with time travel, in science fiction this requires the use of machines, like the TARDIS, and a pseudo-scientific jargon is all that's necessary to explain its existence. This definition even applies to superheroes like Spiderman. Superhero stories allow their creators to demonstrate people's capacity to harm, and to help, on a massive scale, and their characters' ability to fight evil often comes from science. Radioactive spider bites probably won't allow you to swing from webs like Spiderman, but the author still tries to justify a development as rational. On the other hand, superheroes (and villains) can also use "magical" artifacts or powers that exist simply because the author says that's how it is, like the character of Mina Harker in The League of Extraordinary Gentlemen who is a vampire.

In general, science fiction is a genre that requires the reader to accept a reality (or the possibility of a reality) that often contradicts our own, but seeks to aid the reader by accounting for this as the result of future scientific developments. While it is impossible to use this as a universal explanation (there are always exceptions), and it cannot adequately describe the variety involved in the genre, I believe that it does provide a very basic way to classify a work.