HELLA - 1-Pin Water Temperature Gauge Sender as found on many Nissan engines such as the RB and SR series (RB20/25/26 and SR20/20DE/DET)
This is the engine coolant temperature sender that is connected to the water temp gauge on the dashboard (instrument cluster). Note that it is NOT the 2-pin sensor that ECU uses for fueling compensation when the engine is cold etc.