Jeez, people have only been struggling to find a definitive answer to this question for almost thirty years now. Why not ask us something *hard*? ;-)
Here's my answer:
While at their very basest levels, horror's sister genres science fiction and fantasy can often be defined by their tropes (spaceships, magic, advanced technology, sentient non-human lifeforms, etc.) and their settings (space, secondary worlds, the future, magical kingdoms, etc.), horror doesn't have any such ability. Horror can be set anywhere. Horror's supposed tropes (werewolves, vampires, ghosts, serial killers, etc.) appear just as often in fantasy, paranormal romance and mysteries/thrillers. Instead, horror is one of those rare genres -- and I would posit romance is another one -- that can only announce itself by tone. That is, the *intent* behind the use of those tropes. This is what makes horror one of the most inclusive genres, but unfortunately also makes it one of the most difficult to define. The only way to truly define horror, then, is by the emotions a work of literature elicits from the reader. Iconography can play no active role in horror's definition because, as noted above, the tropes can be found just as easily elsewhere. So if the object of a piece of literature is to scare you, to give you a frisson of terror, it's horror. If the tone is one of mounting or pervasive dread and terror, it's horror. But if it has a vampire in it, or a werewolf, or a demon, that does not necessarily make it horror in and of itself.
Any thoughts? Am I on the right track? Do you have another definition? And most importantly, am I accidentally belittling science fiction and fantasy (which certainly wasn't my intent) by hanging their shingles on trope and setting?