Broadly speaking, the term refers to an incident involving multiple victims of gun violence, but there is no official set of criteria or definition for a mass shooting, according to criminology experts and FBI officials. Generally, there are three terms you'll see to describe a perpetrator of this type of gun violence: mass murderer, spree killer, or serial killer. The primary distinction between a mass murderer and a spree killer is that the latter strikes in multiple locations. A serial killer is distinguished by striking over a longer time frame.