A state of emergency is a governmental declaration that may suspend certain normal functions of government, alert citizens to alter their normal behaviors, or order government agencies to implement emergency preparedness plans. It can also be used as a rationale for suspending civil liberties. Such declarations usually come during a time of natural disaster, during periods of civil disorder, or following a declaration of war (in democratic countries, many call this martial law, mostly with non-critical intent). Justitium is its equivalent in Roman law.

In some countries, the state of emergency and its effects on civil liberties and governmental procedure are regulated by the constitution, or a law that limits the powers that may be invoked or rights that may be suspended during an emergency. In many countries, it is illegal to modify the emergency law or constitution during the emergency.

http://en.wikipedia.org/wiki/State_of_emergency