The true definition of horror is a feeling of revulsion, disgust, and dread.
I've only really felt genuine horror from 2 movies. The Boy in the Striped Pajamas and A Serbian film. Neither film would likely be found in the horror section.
Most movies are terror through suspension of disbelief, or creepiness through being creepy, or movies designed to instill fear. "Scary Movies" is a more accurate moniker. Many horror films are merely macabre.
The most horrible thing I ever saw was Muammar Gaddafi get beaten to death on CNN. I didn't like the guy, but it was horrible to watch. It was more like a zombie movie than a zombie movie. There was an utter absence of humanity. I didn't want to see it, but TV has the ability to shock and surprise.
There was a really good interview with Christopher Lee where he outlines how horror movies are not actually horror movies. I can't find it, but it was an excellent interview.
While I was searching for the Christopher Lee interview I found this:
"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".[1] It creates an eerie and frightening atmosphere. Horror is frequently 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."