Wide Range Air Fuel sensor. Also known as Wide Band Air Fuel sensor, or Wide Ratio Air fuel sensor

Gives a wider range of air/fuel ratio information to the ECU for better mixture control.
Early designs of sensors known as Lambda or Oxygen sensors gave a rich / lean indication which meant the ECM could only adjust the injection duration in a fairly crude manor to try and keep the mixture oscillating around the rich/lean centre point. Modern sensors give an actual indication of the fuel ratio so allow the ECM a much finer control over the mixture