Horror is the feeling of dread and anticipation that usually occurs before something frightening is seen, heard, or otherwise experienced. It is also the feeling one gets after coming to an awful realization or experiencing a hideous revelation. (Terror is usually described as the feeling that follows after the experience has occurred.) Horror has also been defined as a combination of terror and revulsion.Compare the experience of waiting for a monster to jump out of a closet (horror) with the experience of actually seeing the scary monster after it has jumped out (terror).Of course, horror can still be felt after the monster(s) etc. is revealed, especially when the word is used in the sense connotating revulsion. A good example is found in the film Rosemary's Baby. The horror in that film is not only anticipatory, but is equal (if not greater) at the moment of revelation at the end of the film and all that it implies for the future.
Horror is also a genre of film and fiction that relies on horrifying images or situations to tell stories and prompt reactions in their audiences.