Proximity

Proximity sensors are used to detect the presence (but not measure the distance) of a particular type of object.

IR proximity sensors will detect any opaque object, capacitive proximity sensors will detect metals and some liquids or powders, inductive proximity sensors also detect metals, and magnetic proximity sensors will detect ferromagnetic objects.

These types of sensors are ideal for keeping track of the position of an object. For example, you could embed a magnetic proximity sensor inside your door frame and put a magnet in the edge of the door to detect whether the door is open or closed.