The characterization of an event as a mass shooting depends upon definition and definitions vary. Under U.S. federal law the Attorney General may on a request from a state assist in investigating "mass killings", rather than mass shootings. The term was originally defined as the murder of four or more people with no cooling-off period but redefined by Congress in 2013 as being murder of three or more people. In "Behind the Bloodshed", a report by USA Today, a mass killing is defined as any incident in which four or more were killed and also includes family killings. A crowdsourced data site cited by CNN, MSNBC, The New York Times, The Washington Post, The Economist, the BBC, etc.,
Mass Shooting Tracker, defines a mass shooting as any incident in which four or more people are shot, whether injured or killed. A noteworthy connection has been reported in the U.S. between mass shootings and domestic or family violence, with a current or former intimate partner or family member killed in 76 of 133 cases (57%), and a perpetrator having previously been charged with domestic violence in 21. The lack of a single definition can lead to alarmism in the news media, with some reports conflating categories of crimes.
In Australia, a 2006 paper defined a mass shooting as "one in which ⩾5 firearm‐related homicides are committed by one or two perpetrators in proximate events in a civilian setting, not counting any perpetrators".
Crime violence research group Gun Violence Archive, whose research is used by all major American media outlets defines Mass Shooting as "FOUR or more shot and/or killed in a single event [incident], at the same general time and location not including the shooter" differentiating between Mass Shooting and Mass Murder [Killing] and not counting shooters as victims.
An act is typically defined as terrorist if it "appears to have been intended" to intimidate or to coerce people; a mass shooting is not, in itself, an act of terrorism. A U.S. Congressional Research Service report explicitly excluded from its definition of public mass shootings those in which the violence is a means to an end, for example where the gunmen "pursue criminal profit or kill in the name of terrorist ideologies".