Coolant Temperature Sensor. A sensor place somewhere in the engines cooling system, usually in the thermostat housing, that sends the vehicles coolant temperature to the PCM/ECU via voltage signal,(Usually a negative thermistor. Meaning as the vehicles temp rises the voltage signal drops). Which then relays a signal to the temp gauge on your instrument cluster. The CTS also may help to put an OBD2 equipped car into closed loop fueling.