There doesn't seem to be an official definition for a "mass shooting" in the United States, but according to the Gun Violence Archive, a mass shooting is described as four or more individuals being shot or killed in the same general time and location.
The F.B.I. has defined "mass killing" and "mass murderer".
The F.B.I. defines a "mass killing" as the killing of three or more people in a public place, but the federal agency also defines a "mass murderer" as someone who has killed four or more people in the same location.
The Gun Violence Archive lists itself as a not-for-profit organization that documents gun violence and gun crime nationally.