A mass shooting is a violent crime in which one or more attackers kill or
injure multiple individuals simultaneously using a firearm. There is no
widely-accepted definition of "mass shooting" and different organizations
tracking such incidents use different definitions. Definitions of mass
shootings exclude warfare and sometimes exclude instances of gang violence,
armed robberies, familicides and terrorism. The perpetrator of an ongoing mass
shooting may be referred to as an active shooter.
In the United States, the country with the most mass shootings, the
Investigative Assistance for Violent Crimes Act of 2012 defines mass killings
as three or more killings in a single incident.