A serial killer is a mass murderer who kills his victims over a period of time in a series of incidents, as opposed to a spree killer that kills a three or more persons, over a short period of time, in a single incident. The term usually carries the implication that the action is being carried out without the color of government or quasi-government authority, in contrast with the UN Convention against Torture definition that implies the term torture implies the acts are carried out with some form of governmental authority.
Some definitions do not use "spree killer", but instead reserve mass murder for one who kills a significant number of people in a short period of time.  Yet other definitions do consider governmental action, such as the Holocaust, both mass and serial killing.
Within the category, one classification categorizes the killers as either organized or disorganized. 
- Robert J. Morton and Mark A. Hilts, ed., Serial Murder — Multi-Disciplinary Perspectives for Investigators, Federal Bureau of Investigation
- Alan C. Brantley amd Robert H. Kosky, Jr. (January 2005), "Serial murder in the Netherlands: a look at motivation, behavior, and characteristics", FBI Law Enforcement Bulletin,