Nobody called anything a horror film until Dracula arrived in 1931
In the silent era, reviewers used the word horror for war scenes, drug stories and melodramas about thieves. According to historian Gary D. Rhodes, the phrase horror film was not part of early cinema at all. The genre only hardened into a recognisable category after Universal's Dracula, which was first sold as a mystery.
Long before cameras, fear had its storytellers: folklore, religious belief and superstition, and Gothic writers such as Mary Shelley, Edgar Allan Poe and Bram Stoker. Early film drew on those sources and on German Expressionism. Yet a 1913 issue of Moving Picture World used horrors to mean a grab bag of lurid stock villains, and a 1914 title, The Hand of Horror, was simply a melodrama about someone robbing his own sister. When Dracula appeared, mystery was the fashionable label, so publicity leaned on it even though the story is plainly supernatural.
Once the 1930s were over, the boundaries loosened again. Kim Newman, writing for the British Film Institute, argued that horror became less a separate genre than an effect that could be dropped into many kinds of story. In the 1940s the New York Times grouped Gaslight and Laura in a new horror cycle, and Siegfried Kracauer filed The Lost Weekend among his terror films. Mark Jancovich has pointed out that horror then functioned almost as a synonym for mystery.
Scholars still offer rival definitions. One textbook stresses deformed characters, suspense and shock, and imagery that swings between the dread of what is hidden and the horror of what is shown. Robin Wood's formula is shorter: normality under threat from a monster. Noël Carroll, puzzling over why anyone watches, concluded that the repulsion must somehow be enjoyable, since the genre is so popular. Some writers prefer to treat forms like the slasher as cycles shaped by money, personnel and marketing rather than as fixed genres.
Fans disagree just as sharply. Some treat Alien as science fiction, others reject it from both camps; some love Freddy Krueger, others follow auteurs like Dario Argento, and still others find Argento too mainstream. Andrew Tudor's answer is that a genre is whatever people collectively believe it to be. Meanwhile the form thrives worldwide, especially in Japan, Korea and Thailand, and has produced both controversy and enormous hits.
Source: Horror film