Mass murder is a legal term used to describe multiple murders committed in one instance or in a short period of time by the same person or people. The exact number of murders that must occur to warrant the use of the term is not well defined, though it is generally considered to be four or more if the murders are committed by an individual or small group. These murders can be committed on a small scale by an individual or small group or on a large scale by a military force, state, or government. Killing sprees, the killing of an individual's family, genocide or strategic bombing of a civilian area can all be referred to as mass murder.
A mass murderer is an individual who is responsible for the murders of a large number of people in a single incident or in a string of closely related incidents. This is particularly clear when a single individual personally commits the murders through a killing spree or through the execution of hostages. Interestingly, however, a political leader who orders a military move that results in a large number of intentional civilian deaths is also commonly considered to be a mass killer, even if he never personally harmed anyone.
Several different categories are used to define and describe mass murder on both a large and small scale, although small-scale mass murder is not as well defined. Such murders are often defined by the setting or by the motivation of the killer. School or workplace shootings, for instance, often result from frustration with coworkers or fellow students or anger over losing one's job. Some mass killers are committed to make some kind of political, philosophical, or moral point. While there are many different possible motives for mass killing, the motives are very seldom sexual in nature, unlike more conventional, small-scale murders or even serial killings.
The categories used to define mass murder on a larger scale are much better defined. Genocide, for instance, is the systematic elimination of a particular religious or cultural group or of all members of a particular nationality. Likewise, political killings involve the intentional elimination of a particular political group within a country. The reigning political group in a country may turn to this method to wipe out rivals. Military mass killings also occur through strategic bombings of civilian areas or through massacres of prisoners or civilians.
Mass murder is a common tactic in terrorism as well. The objective of terrorism is generally to inspire fear in a group of people, and highly-visible and destructive acts with a large death toll are effective at accomplishing this end. Suicide bombings, for instance, commonly occur in areas that are highly populated by civilians.