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.
Since 4 people were shot and killed in Thursday's Casa Grande incident, it fits this definition.
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.