PIR sensor - technical definition

(Passive InfraRed sensor) A device used to detect motion by receiving infrared radiation. When a person walks past the sensor, it detects a rapid change of infrared energy and sends a signal. PIR sensors are used for applications such as automatically turning on lights when someone enters a room or causing a video camera to begin operating.

This passive method is not as reliable as "active" motion sensors that either bounce back a radar signal or transmit light to a photodetector in the distance. See network camera.