Plain English Meaning
A small sensor made from two different metals joined together that produces a tiny electrical signal when heated. The hotter it gets, the bigger the signal, so the cockpit gauge can show the temperature.
Full Definition
Temperature-sensing devices made by joining two wires of different metals at one end. When that junction is heated, a small voltage is produced that varies with temperature. In aircraft, thermocouples are used to measure very high temperatures, such as cylinder head temperature in piston engines and exhaust gas temperature or turbine inlet temperature in turbine engines.
Context Anchor
Seen in turbine engine instrument discussions, especially where temperature or temperature is being measured.