A defense readiness condition (DEFCON) is an alert posture used by the United States Armed Forces. The DEFCON system prescribes five states of alert for the U.S. military, and increase in severity from DEFCON 5 (least severe) to DEFCON 1 (most severe) to match varying military situations.

Defense condition | Exercise term | Description | Color

DEFCON 5 | FADE OUT | Lowest state of readiness | Blue
DEFCON 4 | DOUBLE TAKE | Increased intelligence watch and strengthened security measures | Green
DEFCON 3 | ROUND HOUSE | Increase in force readiness above that required for normal readiness | Yellow
DEFCON 2 | FAST PACE | Next step to nuclear war | Red
DEFCON 1 | COCKED PISTOL | Nuclear war is imminent | White

DEFCON 1 has never been called for.

Source: wikipedia