A
serial killer is a person who
murders three or more people, usually due to
abnormal psychological gratification, with the murders taking place over more than a month and including a significant break (a "cooling off period") between them. Some sources, such as the
FBI, disregard the "three or more" criterion and define serial killing as "a series of two or more murders, committed as separate events, usually, but not always, by one offender acting alone".