https://www.rand.org/research/gun-policy/analysis/essays/mass-shootings.html
What Is a Mass Shooting?
In the 1980s, the Federal Bureau of Investigation (FBI) defined mass murderer as someone who “kills four or more people in a single incident (not including himself), typically in a single location” (Krouse and Richardson, 2015). However, the government has never defined mass shooting as a separate category, and there is not yet a universally accepted definition of the term. Thus, media outlets, academic researchers, and law enforcement agencies frequently use different definitions when discussing mass shootings, which can complicate our understanding of mass shooting trends and their relationship to gun policy. The table below provides examples of the variation in the criteria set by five of the most commonly referenced data sources on mass shootings in the United States.