America does have a standard definition. It's so standard it's in Britannica
"mass shooting, also called active shooter incident, as defined by the U.S. Federal Bureau of Investigation (FBI), an event in which one or more individuals are “actively engaged in killing or attempting to kill people in a populated area. Implicit in this definition is the shooter’s 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.” 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."
