Active shooter or active killer describes the perpetrator of a type of mass murder marked by rapidity, scale, randomness, and often suicide. The Federal Bureau of Investigation defines an active shooter as "one or more individuals actively engaged in killing or attempting to kill people in a populated area.", excluding self-defense, gang or drug violence, crossfire, and domestic disputes. The United States Department of Homeland Security defines an active shooter as "an individual actively engaged in killing or attempting to kill people in a confined and populated area; in most cases, active shooters use firearms and there is no pattern or method to this selection of victims."

Most incidents occur at locations in which the killers find little impediment in pressing their attack. Locations are generally described as soft targets, that is, they carry limited security measures to protect members of the public. In most instances, shooters die by suicide, are shot by police, or surrender when confrontation with responding law enforcement becomes unavoidable, and active shooter events are often over in 10 to 15 minutes. "According to New York City Police Department (NYPD) statistics, 46 percent of active shooter incidents are ended by the application of force by police or security, 40 percent end in the shooter's suicide, 14 percent of the time the shooter surrenders, and in less than 1 percent of cases the violence ends with the attacker fleeing."