This article has multiple issues. Please help improve it or discuss these issues on the talk page. (Learn how and when to remove these template messages)
A serial killer is typically a person who murders three or more people, in two or more separate events over a period of time, for primarily psychological reasons. There are gaps of time between the killings, which may range from a few days to months, or many years. This list shows serial killers from the 20th century to present day by number of victims. In many cases, the exact number of victims assigned to a serial killer is not known, and even if that person is convicted of a few, there can be the possibility that they killed many more.
Organization and ranking of serial killings is made difficult by the complex nature of serial killers and incomplete knowledge of the full extent of many killers' crimes. To address this, multiple categories have been provided in order to more accurately describe the nature of certain serial murders. This is not a reflection of an individual's overall rank, which may or may not vary depending on personal opinion concerning the nature and circumstances of their crimes. The fourth column in the table states the number of victims definitely assigned to that particular serial killer, and thus the table is in order of that figure. The fifth column states the number of possible victims the killer could have murdered. Some of these crimes are unsolved, but were included because they are the work of a serial killer, despite nobody being caught.
A person who murders 3+ people over a period of > 30 days, with an inactive period between each murder, and whose motivation for killing is largely based on psychological gratification.
Serial murder is the killing of three or more people over a period of more than 30 days, with a significant cooling-off period between the murders [...] The baseline number of three victims appears to be most common among those who are the academic authorities in the field. The time frame also appears to be an agreed-upon component of the definition.
Three killings seem to be required in the most popular operational definition of serial killing since they are enough to provide a pattern within the killings without being overly restrictive.
In general, most experts on serial murder require that a minimum of three murders be committed at different times and usually different places for a person to qualify as a serial killer.
Most experts seem to agree, however, that to qualify as a serial killer, an individual has to slay a minimum of three unrelated victims.