Hall sensor

The function of a Hall sensor is based on the principle of the Hall effect named after its discoverer E. H. Hall.

It means that a voltage is generated transversely to the current flow direction in an electric conductor (the Hall voltage), if a magnetic field is applied perpendicularly to the conductor. As the Hall effect is most pronounced in semiconductors, the most suitable Hall element is a small platelet made of semiconductive material.

Or, in plain English, a magnetically activated semicondutor switch or sensor.