Slasher

The term "slasher" generally describes that genre of cinematic horror which devotes ample screen time to bloody, brutal attacks perpetrated by a serial killer using a hand-held weapon or tool designed to cut, hack, or slash, such as a knife, razor blade, sword, machete, hatchet, axe, and even the occasional chainsaw. Among the subgenre's recurring elements, we would cite:

1. the victims tend to be young, usually a group of stereotypical teenagers. The canonical formula calls for a female protagonist to serve as the "final girl", a sort of last survivor who must ultimately confront and defeat the monster. Meanwhile, the girl's friends, relatives, and companions merely serve as victims to be slaughtered somewhere along the line;
2. the film features an elevated body count (it's not unusual to hit double digits);
3. the plot tends to be relatively focused as far as time and location. In fact, the action often unfolds in a single day (or night) and is generally confined to one distinct setting - popular choices include summer camps, boarding schools, and colleges (+);
4. there's an underlying sanctimoniousness, stemming from the fact that the victims are mainly depicted as "sinful" (horny teens are frequently the first to die, often while in the very throes of passion);
5. the killer tends to have distinctive characteristics: Their face is generally covered and/or disfigured, and they tend to use a signature (non-firearm) murder weapon, ranging from the classic butcher knife to a machete, axe, or even a billhook, pitchfork, or shears. Sometimes the choice can be a tad bit more creative, such as the occasional nail gun or chainsaw...

 
Although it's possible to identify several films prior to 1978 that at least partially satisfy the above criteria, "Halloween" is conventionally considered the single work that gave rise to the slasher film. The choice is pretty much based on the extraordinary impact that John Carpenter's film had on the genre, given that its unexpected success prompted hundreds of derivative products to be produced in the years immediately following its release.