The definition of a mass shooting varies depending on the source. The FBI defines an active shooter as an individual actively engaged in killing or attempting to kill people in a populated area. The FBI has not set a minimum number of casualties to qualify an event as a mass shooting, but the U.S. statute (the Investigative Assistance for Violent Crimes Act of 2012) defines a “mass killing” as “3 or more killings in a single incident”. The Gun Violence Archive defines a mass shooting as “four or more people are shot or killed in a single incident, not including the shooter”. Mother Jones defines a mass shooting as an indiscriminate rampage in a public place, resulting in three or more victims killed by the attacker, excluding gang violence, armed robbery, and attacks by unidentified perpetrators. Everytown for Gun Safety, a gun control advocacy group, defines mass shootings as any incident in which four or more people are shot and killed, excluding the shooter.

In general, a mass shooting is a violent crime in which an attacker kills or injures multiple individuals simultaneously using a firearm. The term “mass shooting” can refer to firearm-related events with various outcomes depending on which definition is being used. News reports on the number of mass shootings in the US are likely to reference the Gun Violence Archive, a nonprofit that defines a mass shooting as an event with a “minimum of four victims shot, either injured or killed, not including any shooter who may also have been killed or injured in the incident”.