A serial killer is a murderer who has killed several people. Many horror movies feature terrifying serial killers.
Officially, a serial killer is a person who murders more than three people, although not all at the same time. Law enforcement officers who study and pursue serial killers believe that these kinds of murders are performed for some psychological reason, usually very deliberately and with "down time" between each killing. The word serial, or "one in a series," has been used to talk about murder since the 1960s, though the term serial killer was first used in 1981.