A serial killer is a person who murders three or more people in a period of over a month, with a "cooling down" time between murders. The murders must be separate events, which are most often driven by a psychological thrill or pleasure. Serial killers often lack empathy and guilt, and most often become egocentric individuals; these characteristics classify certain serial killers as psychopaths. Some commonly found characteristics of serial killers include varying degrees of mental illness or psychopathy, which may contribute to their homicidal behavior. They may also exhibit a "mask of sanity" to hide their true psychopathic tendencies and appear normal, even charming.

It is important to note that a serial killer is a distinct classification that differs from that of a mass murderer, spree killer, or contract killer. A mass murderer is someone who kills multiple people in a single incident with no cooling off period. A spree killer is someone who kills two or more victims in a short time in multiple locations. A contract killer is someone who is hired to kill someone else.

In summary, a serial killer is defined by the following characteristics:

• Murders three or more people in a period of over a month, with a "cooling down" time between murders
• The murders must be separate events, which are most often driven by a psychological thrill or pleasure
• Often lacks empathy and guilt, and most often become egocentric individuals; these characteristics classify certain serial killers as psychopaths
• May exhibit a "mask of sanity" to hide their true psychopathic tendencies and appear normal, even charming.