Duress code
In military / security communication, a duress code is a simple word or phrase used during normal conversation to alert other personnel that an authorized person is under duress. A duress code requires planning and rehearsal to ensure an appropriate response. This code is changed frequently to minimize compromise.
In Day 5, Hour 3, Jack Bauer uses a duress code to alert CTU (Counter Terrorist Unit) his position has been compromised and infiltration of the airport should be aborted. Jack's duress code is 'flank two'.
Alarm Systems
Some home and property alarm systems have duress codes. Often the last two digits of the reset code are switched around. Entering the code when under duress from an assailant often triggers a silent alarm, alerting police or security personel in a covert manner.