Mass murder is the act of murdering a number of people, typically simultaneously or over a relatively short period of time and in close geographic proximity.^[1][2] The FBI defines mass murder as murdering four or more people during an event with no "cooling-off period" between the murders. A mass murder typically occurs in a single location where one or more people kill several others.^[3][4]

A mass murder may be committed by individuals or organizations whereas a spree killing is committed by one or two individuals. Mass murderers differ from spree killers, who kill at two or more locations with almost no time break between murders and are not defined by the number of victims, and serial killers, who may kill people over long periods of time. Mass murder is a hypernym of genocide, which requires additional criteria.

Many terrorist groups in recent times have used the tactic of killing many victims to fulfill their political aims. Such incidents have included the:

Certain cults, especially religious cults, have committed a number of mass killings and mass murder-suicides. These include Jim Jones' Peoples Temple in Jonestown, Guyana, where 919 people died in 1978; the Order of the Solar Temple in Canada, Switzerland, and France, where 75 died in 1994, 1995, and 1997; Shoko Asahara's Aum Shinrikyo, which killed 12 in Tokyo, Japan, in 1995; Marshall Applewhite's Heaven's Gate in San Diego, California, where 39 died in 1997; and the Movement for the Restoration of the Ten Commandments of God in Uganda, where 778 died in 2000.

Mass murderers may fall into any of a number of categories, including killers of family, of coworkers, of students, and of random strangers. Their motives for murder vary.^[5] A notable motivation for mass murder is revenge, but other motivations are possible, including the need for attention or fame.^[6][7][8]

Acting on the orders of Joseph Stalin, Vasili Blokhin's war crime killing of 7,000 Polish prisoners of war, shot in 28 days, is notable as one of the most organized and protracted mass murders by a single individual on record.^[9]

Analysis of the Columbine High School massacre and other incidents where law enforcement officers waited for backup has resulted in changed recommendations regarding what victims, bystanders, and law enforcement officers should do. Average response time by law enforcement to a mass shooting is typically much longer than the time the shooter is engaged in killing. While immediate action may be extremely dangerous, it may save lives which would be lost if victims and bystanders involved in the situation remain passive, or law enforcement response is delayed until overwhelming force can be deployed. It is recommended that victims and bystanders involved in the incident take active steps to flee, hide, or fight the shooter and that law enforcement officers present or first arriving at the scene attempt immediately to engage the shooter. In many instances, immediate action by victims, bystanders, or law enforcement officers has saved lives.^[10]

Commentators have pointed out that there are a wide variety of ways that homicides with more than several victims might be classified. Such incidents can be, and have been even in recent decades, classified many different ways including "as a mass shooting; as a school shooting; as mass murder; as workplace violence...; as a crime involving an assault rifle; as a case of a mentally ill person committing acts of violence; and so on."^[11]

How such rarely occurring incidents of homicide are classified tends to change significantly with time. "In the 1960s and 1970s,... it was understood that the key feature of [a number of such] cases was a high body count. These early discussions of mass murder lumped together [a variety of] cases that varied along what would come to be seen as important dimensions:

In the late decades of the twentieth century and early years of the 2000s, the most popular classifications moved to include method, time and place.

While such classifications may assist in gaining human meaning, as human-selected categories, they can also carry significant meaning and reflect a particular point of view of the commentator who assigned the descriptor.^[11]