An abbreviation of Passive Infrared: a system commonly found in intruder detection devices. A PIR system will monitor an area for any changes in the infrared radiation such as might be caused by the body heat of a human intruder. The "passive" aspect refers to the fact that the detector does not need to emit any signals of its own, it simply monitors incoming radiation. PIR systems have also been used by the military, both in missile and counter-missile technology.

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