(Borrowed from Wikipedia)
In the U.S. military, the term REDCON is short for Readiness Condition and is used to refer to a unit's readiness to respond to and engage in combat operations. There are four REDCON levels, as described below in this excerpt from Army Field Manual 71-1.
Full alert; unit ready to move and fight.
Full alert; unit ready to fight.
Reduced alert.
Minimum alert.