The National Security and Homeland Security Presidential Directive (National Security Presidential Directive NSPD 51/Homeland Security Presidential Directive HSPD-20, sometimes called simply "Executive Directive 51" for short), created and signed by the President of the United States on May 4, 2007, is a Presidential Directive which claims power to execute procedures for continuity of the federal government in the event of a "catastrophic emergency".
Such an emergency is construed as "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."
The unclassified portion of the directive was posted on the White House website without any further announcement or press briefings. The presidential directive says that, when the president considers an emergency to have occurred, an "Enduring Constitutional Government" comprising of "a cooperative effort among the executive, legislative, and judicial branches of the Federal Government, coordinated by the President," will take the place of the nation's regular government. The directive, created by the president, claims that the president has the power to declare a catastrophic emergency.
It does not however, specify who has the power to declare the emergency over.