Sure, we have a definition of the horror genre -
A genre of literature, which is intended to, or has the capacity to frighten, scare, or startle its readers by inducing feelings of horror and terror. Literary historian J. A. Cuddon has defined the horror story as "a piece of fiction in prose of variable length... which shocks or even frightens the reader, or perhaps induces a feeling of repulsion or loathing." It creates an eerie and frightening atmosphere. Horror is usually supernatural, though it can be non-supernatural. Often the central menace of a work of horror fiction can be interpreted as a metaphor for the larger fears of a society. (Wikipedia)
(Note: Yes, I cite Wikipedia in those cases where there are enough references to verify its accuracy. Sue me.)
Still, sometimes I look at the books I've read, or movies I've seen, and think, "It may have been a mystery/thriller/romance/literary fiction, but I was terrified. Why isn't it in the horror section?"
A.C. Doyle's Professor Challenger stories always seemed to horrify me at some point, but they are listed as fantasy or science fiction. H. Rider Haggard's SHE has a most scary ending - the shower of eternal youth and beauty must only be used once. It is also listed as science fiction. Even the Indiana Jones movies have a horror element to them. I mean, heads explode, faces melt, hearts are ripped from living bodies... the list goes on. It's billed as action/adventure, but trust me, I was pretty repulsed by the man aging to a skeleton by drinking from the wrong cup.
As for works that are considered in the horror category, we shudder when we think of Poe's writings. I don't know about you, but reading about people being buried alive, plagues, and madness induced a feeling of ickiness in me.
Stephen King is definitely horror. His stories frighten, scare, startle, and repulse me. I used to read King voraciously. I still love his non-horror novellas, STAND BY ME and THE SHAWSHANK REDEMPTION. But I stopped reading him after Pet Sematary because he killed a child. I let him have a pass with Cujo. I guess with me, it's two strikes and you're out.
Dean Koontz has a horror edge to his tales, but he is not horror to me because at the end of the book, his heroes survive and evil is vanquished. I can breathe easily and sleep at night, knowing Mr. Koontz has saved the earth from the enemy he created.
So my own definition of horror would have to include an ending which does not promise a bright tomorrow. The hero might survive, but so would the villain. Even if the evil is eradicated, the cost is terrible.
(Aside: If horrific means having the power to horrify, why doesn't terrific mean having the power to terrify?)
What is your definition of horror? Do you read books or watch movies that scare you, even if you don't like the horror genre? What makes the difference?
Oh, and BTW, which one of Koontz's books would you like to see as a movie?