The term "mass shooting" can refer to firearm-related events with various outcomes depending on which definition is being used. There is no widely-accepted definition of "mass shooting" and different organizations and sources use different criteria. Here are some of the most commonly used definitions:

• The Gun Violence Archive 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".
• The FBI defines an active shooter as an individual actively engaged in killing or attempting to kill people in a populated area. Implicit in this definition is the shooters use of a firearm. The FBI has not set a minimum number of casualties to qualify an event as a mass shooting, but 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".
• 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.
• CBS defines a mass shooting as an event involving the shooting (not necessarily resulting in death) of five or more people (sometimes four) with no cooling-off period.

Its important to note that the definition used can affect the number of incidents that are counted as mass shootings. For example, the Gun Violence Archives definition tends to result in higher numbers than other lists because it includes all victims and not just fatalities.