PIR is shortened for Passive Infrared. PIR sensor is an electronic sensor that measures infrared (IR) light radiating from objects in its field of view.
The PIR sensor (passive infrared sensor) detects changes in the amount of infrared radiation impinging upon it, which varies depending on the temperature and surface characteristics of the objects in front of the sensor. When an object, such as a human, passes in front of the background, such as a wall, the temperature at that point in the sensor's field of view will rise from room temperature to body temperature, and then back again.
The sensor converts the resulting change in the incoming infrared radiation into a change in the output voltage, and this triggers the detection. Objects of similar temperature but different surface characteristics may also have a different infrared emission pattern, and thus moving them with respect to the background may trigger the detector as well. This function most often used in motion detection to decrease the false alarm.