Because the feeling of horror is subjective, defining horror is problematic. Though most people will agree on the feeling in a broad sense, each person may recognize nuances not perceived by others. Thus there may be as many flavors of horror as there are people. Therefore, instead of trying to arrive at a precise definition of a nebulous topic, here I shall collate quotations on and definitions of horror from various sources to allow readers to form their own subject definition. Please consider this page a living document that will grow as I add more to it with time. Its organization is mostly random with the most recent additions being at the bottom of the page. Brief material I shall quote here. I will provide links to longer material.
Etymology of the word “Horror” from www.etymonline.com
horror (n.) 
early 14c., from Old French horror (12c., Modern French horreur) and directly from Latin horror “dread, veneration, religious awe,” a figurative use, literally “a shaking, trembling, shudder, chill,” from horrere “to bristle with fear, shudder,” from PIE root *ghers- “to bristle” (cf. Sanskrit harsate “bristles,” Avestan zarshayamna- “ruffling one’s feathers,” Latin eris (genitive) “hedgehog,” Welsh garw “rough”). As a genre in film, 1934. Chamber of horrors originally (1849) was a gallery of notorious criminals in Madame Tussaud’s wax exhibition.
Here is an interesting discussion of the etymology of Horror that is too long to post here: www.ligotti.net
Here is another extensive but fascinating article on the etymology of “horror”: writinghorrorfiction.blogspot.com.
For modern definitions of horror visit the entries for “horror” at Merriam-Webster or Oxford Dictionaries.
Synonyms can be found at Collins Dictionary.
Here is a good article on the distinction between “horror” and “terror” at Wikipedia.
Stephen King
at Comicon, 2007
Photo by Penguino
“The 3 types of terror: The Gross-out: the sight of a severed head tumbling down a flight of stairs, it’s when the lights go out and something green and slimy splatters against your arm. The Horror: the unnatural, spiders the size of bears, the dead waking up and walking around, it’s when the lights go out and something with claws grabs you by the arm. And the last and worse one: Terror, when you come home and notice everything you own had been taken away and replaced by an exact substitute. It’s when the lights go out and you feel something behind you, you hear it, you feel its breath against your ear, but when you turn around, there’s nothing there…” ― Stephen King as quoted in Goodreads.com
If you would like a truly intellectual definition of horror, check out this article by Daniel Shaw at http://www.film-philosophy.com: “A Humean Definition of Horror“.
Talkingphilosophy.com has the article “Tragically Defining Horror” posted by Mike LaBossiere .
Another scholarly article on the definition of horror is to be found at The American International Journal of Contemporary Research: “The Genre of Horror” by Mgr. Viktória Prohászková.
EricHoefler.com has the article Noel Carroll and the Definition of Horror.
Here is the Horror Writers Association definition of horror fiction
Phil Slattery, Writer, Publisher 