Question:What is a passive infrared sensor?
A sensor is a device that absorbs some kind of radiation and produces an electrical signal in response to the radiation than can be used in some way.
Passive sensors are ones that just wait for the infrared radiation to fall on them.
There is another kind that you do not need to know about that would be called an active sensor because they emit the infrared radiation themselves and detect what reflects back.
So Passive infrared sensors are sensors that detect infrared radiation passively.