A serial killer is typically a person who murders three or more people, usually in service of abnormal psychological gratification, with the murders taking place over more than a month and including a significant period of time between them. Different authorities apply different criteria when designating serial killers. For example, while most authorities set a threshold of three murders, others extend it to four or lessen it to two. The Federal Bureau of Investigation (FBI) defines serial killing as "a series of two or more murders, committed as separate events, usually, but not always, by one offender acting alone".
Although psychological gratification is the usual motive for serial killing, and most serial killings involve sexual contact with the victim, the FBI states that the motives of serial killers can include anger, thrill-seeking, financial gain, and attention seeking. The murders may be attempted or completed in a similar fashion. The victims may have something in common, for example, demographic profile, appearance, gender or race. A serial killer is neither a mass murderer, nor a spree killer, although there may be conceptual overlaps between serial killers and spree killers.