Hall Effect Sensor
A sensor that can detect a magnetic field.
In phones, its most common use is detecting the proximity of a magnet embedded in an attached accessory, such as a flip cover or VR goggles.
For example, with a flip cover, a small magnet embedded in the flip cover is positioned so that it aligns with -- and triggers -- the Hall Effect sensor when the cover is closed. When the sensor is triggered by the magnet, the screen shuts off or switches to a special idle mode.