According to the California Hospital Association, “An active shooter is defined as an individual who appears to be actively engaged in killing or attempting to kill people in the hospital or on the hospital campus. In most cases active shooters use a firearm(s) and display no pattern or method for selection of their victims. In some cases active shooters use other weapons and/or improvised explosive devices to cause additional victims and act as an impediment to police and emergency responders. These improvised explosive devices may detonate immediately, have delayed detonation fuses, or detonate on contact.”
The California Hospital Association has made their Active Shooter guidance documents freely available on their website.