A thermocouple is a temperature sensor that operates on the principle of the Seebeck effect. It consists of two different metal wires joined at one end, forming a junction. When the junction is exposed to heat, a voltage is produced due to the difference in electrical potential between the two metals. This voltage is proportional to the temperature difference between the junction and the other end of the wires. Measuring this voltage provides an accurate measurement of the temperature at the junction.