As a fantasy writer, having a non-fantasy reader look at your work is kind of like sky-diving—for someone who is afraid of heights and planes and trampolines. And staircases with more than three steps.
Despite the ulcer-inciting anxiety this produces, I regularly ask non-fantasy people to beta read for me. And a question I get a lot—no, like a lot—goes something like this: "what's all this science fantasy garbage, anyway?"
Besides the obvious answer to that question (adamantium-grade awesome), I'm going to spend some time agonizing over the slippery edges of the genre, and I’m going to drag you along for the ride … because I’m sadistic like that.
First, Some Super Flexy Definitions:
Fantasy generally assumes the impossible is possible due to magic or some other largely inexplicable feature. Examples: Lord of the Rings, Harry Potter, Twilight (oh yeah, I went there).
Science fiction often suggests that what seems impossible can be made possible by science. Examples: Frankenstein, The Time Machine, Jurassic Park.
The real difference here is that dead bodies are resurrected in Twilight by some sort of inexplicable and unmeasurable "vampire-ness," whereas in Frankenstein those dead bodies (well, parts of dead bodies) are resurrected through electricity. In Frankenstein, there is an explanation for the impossible becoming possible, one that correlates with at least some scientific facts, and that correlation allows the story to take place.
Science fantasy is a greedy genre that tends to employ tropes from traditional fantasy fiction but seeks to create scientific correlations. If Mary Shelley had decidedly portrayed Frankenstein’s monster as a zombie (or any other specific fantasy trope), then the work would be much more recognizable as science fantasy rather than science fiction or horror.
A Totally Random Example:
Rather than representing witches as cranky moss-ridden women with supernatural powers who may or may not eat children, a science fantasy might offer any of the following explanations: witches are actually cyborgs (who eat moss and maybe children); witches are actually genetically altered humans from the future (who would never eat children, but—due to their futuristic dietary needs—often eat moss); witches are intra-dimensional humanoids who learned, through trial and error, that children are not a suitable dietary staple.
There are quite a few alternatives to this, but the point is that witches and their powers are explained through some branch of science—robotics, genetic modification, the Many-Worlds interpretations of quantum mechanics.
Regardless of whether witches eat children or moss or both, their existence is introduced or explained through one or several branches of science.
Lastly: some science fantasy might just feature unicorns shooting laser guns . . . which is pretty much the best kind.
A Post Script: Just in case you’re one of the ultra-rare weirdos who is fascinated by all this genre talk, I’m including a link to an excellent piece by Randy Henderson at Fantasy Magazine.