A serial killer is a person who murders three or more people, usually doing so for abnormal psychological gratification, thrill-seeking, attention, or financial gain. The definition of a serial killer differs between authorities where the period of time is changed or the number of murders. The FBI defines a serial killer as someone committing two or more murders as separate events and usually done by one person acting alone.
Serial killings are often attempted in a similar fashion, and victims often have something in common, such as similar demographic profiles. Ted Bundy, for example, targeted attractive, college-aged females.
Serial killers have some common characteristics among them, such as varying degrees of psychopathy, a history of abuse by a family member, may have a fascination with setting fires, or are involved in sadistic activities such as torturing animals.