A mass shooting is an event in which one or more individuals are actively engaged in killing or attempting to kill people in a populated area, and the shooter(s) use a firearm. The FBI has not set a minimum number of casualties to qualify an event as a mass shooting, but the Investigative Assistance for Violent Crimes Act of 2012 defines a "mass killing" as "3 or more killings in a single incident". For the purposes of this article, both sets of criteria will be applied to the term mass shooting, with the distinction that the shooter or shooters are not included in any fatality statistics.

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".

It is important to note that all mass shootings are criminal acts, but there are multiple kinds of shootings, which account for the different numbers reported. The Gun Violence Archive does not exclude any shooting incident as long as it meets the total number of people shot or killed, so their numbers tend to be higher than other lists.

In summary, a mass shooting is an event in which one or more individuals are actively engaged in killing or attempting to kill people in a populated area, and the shooter(s) use a firearm. The exact definition of a mass shooting can vary depending on the source, but it generally involves multiple victims being shot or killed in a single incident.