A serial killer is typically defined as an individual who has killed three or more people over a period of more than a month, with down time between the murders, and whose motivation for killing is usually based on psychological gratification. Some sources disregard the "three or more" criteria, and define the term as "a series of two or more murders, committed as separate events, usually, but not always, by one offender acting alone" or, including the vital characteristics, a minimum of two murders. Often, a sexual element is involved in the killings, but the FBI states that motives for serial murder include "anger, thrill, financial gain, and attention seeking". The murders may have been attempted or completed in a similar fashion and the victims may have had something in common, for example, occupation, race, appearance, sex, or age group.
Serial killers are not the same as mass murderers, nor are they spree killers; they all must kill 3 or more people. Mass murders kill 3 or more people but in one event at one location. Spree killers kill 3 or more people but it is confined to a thirty day period. To be considered a serial killer, they must kill 3 or more people over more then 30 days and have a cooling off period between the killings .
