National Terrorism Advisory System (NTAS)
The National Terrorism Advisory System (NTAS) replaced the color-coded system of heightened public alerts issued by the Department of Homeland Security.
See a sample alert in PDF.
Description from the NTAS site:
This system will more effectively communicate information about terrorist threats by providing timely, detailed information to the public, government agencies, first responders, airports and other transportation hubs, and the private sector.
It recognizes that Americans all share responsibility for the nation’s security, and should always be aware of the heightened risk of terrorist attack in the United States and what they should do.