Do a Google search for the term "mass shooting" and the results just might surprise you. In the U.S., there was approximately one mass shooting per day in 2017. This year is off to a slightly quieter start, but not by much.

How a mass shooting or killing is identified depends on the criteria used. While there is no single, broadly recognized definition of a mass shooting, following the shooting at Sandy Hook Elementary School in Newtown, Conn., Congress defined "mass killings" as "three or more killings in a single incident." The FBI uses the term "mass murder" and defines it as an event where four or more individuals are killed in a single event, usually in one location or within multiple sites that are in close proximity of each other (e.g., different offices or floors in particular setting like a school, office or mall). However, the FBI does not have formal definition for a mass shooting.

Mass Shooting Tracker, which uses multiple sources to track shooting incidents, defines a mass shooting as "an incident where four or more people are shot in a single shooting spree." The organization believes the differentiation is critical because even though victims may not die, they were shot and could have lasting repercussions from the shooting event.