(Borrowed from Wikipedia)
The Defense Readiness Condition (DEFCON) is a measure of the activation and readiness level of the United States Armed Forces. It describes progressive postures for use between the Joint Chiefs of Staff and the commanders of unified commands. DEFCONs are matched to the situations of military severity. Standard peacetime protocol is DEFCON 5, descending in increasingly severe situations. DEFCON 1 represents expectation of actual imminent attack, and is not known to have ever been declared. During the Cold War, DEFCON 1 was feared because it would most likely precede an all-out nuclear war.
This is the condition used to designate normal peacetime military readiness. An upgrade in military preparedness is typically made by the Joint Chiefs of Staff and announced by the United States Secretary of Defense.
This refers to normal, increased intelligence and the heightening of national security measures.
This refers to an increase to force readiness above normal. Radio call signs used by American forces change to currently classified call signs.
This refers to a further increase in force readiness just below maximum readiness. The most notable time it was declared was during the Cuban Missile Crisis, although the declaration was limited to Strategic Air Command. It is not certain how many times this level of readiness has been reached.
This refers to maximum readiness. It is not certain whether this has ever been used, but it is reserved for imminent or ongoing attack on US military forces or US territory by a foreign military power.
The DEFCON level is controlled primarily by the President and the Joint Chiefs of Staff, and each DEFCON level defines specific security, activation and response scenarios for the troops in question.
Different branches of the armed forces (like the Army, Navy, Air Force) and different bases or command groups can be activated at different defense conditions.