You're dubious of Wikipedia -- FilmSite.org is always a good avenue to explore for definitions. I belong to Encyclopedia Britannica but haven't looked up the article on horror films. I've been to more than one lectures, usually at film schools, where this genre is explored thoroughly.
Here's the link to FilmSite's essay:
http://www.filmsite.org/horrorfilms.html
The problem is always going to be in the hybrid of horror mixed with sci-fi, fantasy, crime, and other genres. "The Silence of the Lambs" is a good example of a crime film but is most often noted as a horror film. There is a lot of the horror element in the Lord of the Rings trilogy. "Alien" is a sci-fi horror movie as well as "Frankenstein." It depends on your beliefs if "Dracula" is, in fact, a horror fantasy flick.