A tripwire is a passive triggering mechanism, usually/originally employed for military purposes, although its principle has been used since prehistory for methods of trapping game.
Typically, a wire or cord is attached to some device for detecting or reacting to physical movement. From this basic meaning, several extended and metaphorical uses of the term have developed.
Military usage may designate a tripwire as a wire attached to one or more mines — normally bounding and fragmentation type mines — in order to increase their activation area. Alternatively, tripwires are frequently used in boobytraps, whereby a tug on the wire (or release of tension on it) will detonate the explosives.
Soldiers sometimes detect the presence of tripwires by spraying the area with Silly String. If the string falls to the ground there are no tripwires. If there is a tripwire, the string will be suspended in the air without pulling the wire. It is being used by U.S. troops in Iraq for this purpose.