Bosch 2-Pin Water Temperature Sensor (ECU) as found in most RB20 / RB25 / RB26 engines as well as several SR18 / SR20, VQ-series engines and more.
This is the engine coolant temperature (ECT) sensor that the ECU uses to make adjustments to fueling and ignition as the engine coolant temperature changes.
Note that this is NOT the sender that is connected to the water temp gauge on the dashboard. Can be found here: Water Temp Gauge Sensor