Violent attacks on U.S. civilians, including mass shootings, are sadly nothing new. But why are some of these tragedies labeled “terrorism” and others not? It turns out that there are multiple and sometimes conflicting definitions of the term. Some definitions require that terrorists be associated with foreign terrorist organizations, though some critics disagree with applying the terrorist label only to perpetrators with ties to foreign, rather than homegrown, ideologies. This Lowdown lesson will explore the ever-evolving definition of terrorism in the context of recent and past attacks.