The following is the agreed-upon definition of an Active Shooter by U.S. Government Agencies including the White House, U.S. Department of Justice/FBI, U.S. Department of Education, and Department of Homeland Security/Federal Emergency Management Agency: 

An Active Shooter is “an individual(s) actively engaged in killing or attempting to kill people in a populated area.“
In most cases of mass casualty attacks, the actors use firearm(s), but it is important to be aware that other weapons are sometimes used such as explosives, cutting instruments and vehicles.

Active Shooter situations are unpredictable and evolve quickly. Because most Active Shooter situations are often over within five (5) minutes before Law Enforcement arrives on the scene, individuals must be prepared both mentally and physically to deal with an Active Shooter situation.

In an Active Shooter incident, civilians will often have to make life and death decisions and as a result, they should be prepared through training on the potential decisions they may face.