(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.

Motion detection is a key feature of security systems in residential and commercial environments. Until recently, systems have relied heavily on closed circuit television (CCTV) and passive infrared ...

Most of us would probably expect to see a PIR sensor, but this light is motion sensing with microwaves. We have seen such units tested in the past, which links to a video [hclxing] also references.