NSPD 51
http://www.whitehouse.gov/news/releases/2007/05/20070509-12.html

(b) "Catastrophic Emergency" means any incident, regardless of location, that results in extraordinary levels of mass casualties, damage, or disruption severely affecting the U.S. population, infrastructure, environment, economy, or government functions

THAT'S ALL I AM GOING TO SAY ABOUT THAT.