The defense readiness condition (DEFCON) is an alert state used by the United States Armed Forces. The DEFCON system was developed by the Joint Chiefs of Staff (JCS) and unified and specified combatant commands. After North American Aerospace Defense Command (NORAD) was created, the command used different readiness levels (Normal, Increased, Maximum) subdivided into eight conditions, e.g., the "Maximum Readiness" level had two conditions "Air Defense Readiness" and "Air Defense Emergency". DEFCON was then created in 1959 for use by all military commands and the national government as a joint scale of emergency measurement and alertness.
Raise security levels to face real yet still uncertain dangers, through measures that are local and minimally disruptive of normal activity, while preparing to switch to "orange" or "red" within a few days.
Possible threat / prevent terrorist action
Take measures against plausible risks of terrorist action, including the use of means that are moderately disruptive to normal public activities, while preparing to switch to "red" or "scarlet" on short notice where possible.
High chance of threat / prevent serious attack
Take measures against a proven risk of one or more terrorist actions, including measures to protect public institutions and putting in place appropriate means for rescue and response, authorizing a significant level of disruption to social and economic activity.
Definite threat / prevent major attack
Notification of a risk of major attacks, simultaneous or otherwise, using non-conventional means and causing major devastation; preparing appropriate means of rescue and response, measures that are highly disruptive to public life are authorized.
The post 9/11 UK equivalent to DEFCON (UK 'Threat levels')Edit
An attack is expected imminently.
Maximum protective security measures to meet specific threats and to minimise vulnerability and risk. Critical may also be used if a nuclear attack is expected.
An attack is highly likely.
Additional and sustainable protective security measures reflecting the broad nature of the threat combined with specific business and geographical vulnerabilities and judgments on acceptable risk.
An attack is a strong possibility.
An attack is possible, but not likely.
Routine protective security measures appropriate to the business concerned.