Homeland Security Advisory System. A system of color-coded rankings to warn the nation of terrorist threats posed against America. The five-level warning system is designed to provide guidance to law enforcement and other public agencies, citizens and the private sector. HSAS does this by establishing five threat level conditions with associated protective measures that should be taken by government agencies in response. For every level of threat, there is a coordinated level of preparedness. See Department of Homeland Security, Office of Homeland Security.