I might/might not be one.
But it definitely is a proximity sensor. Unless yours is an Apple device, in which case, it might be an angle sensor.

The term "Hall sensor" would refer to the tech used in it, whereas the term "proximity sensor" refers to its function.
It could be using any other proximity sensing technique too and it would still be a proximity sensor.

If the Dexter actor is near it, the screen goes off

On thinkpads it is, there is a magnet on the bottom.