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, and familicides. 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.[1] A Congressional Research Service report from 2013 specifies four or more killings on indiscriminate victims while excluding violence committed as a means to an end, such as robbery or terrorism.[2] Media outlets such as CNN and some crime violence research groups such as the Gun Violence Archive define mass shootings as involving "four or more shot (injured or killed) in a single incident, at the same general time and location, not including the shooter".[3] Mother Jones magazine defines mass shootings as indiscriminate rampages killing three or more individuals excluding the perpetrator, gang violence, and armed robbery.[4][5] An Australian study from 2006 specifies five individuals killed.[6]

there is no one definition