What's this sensor called? Found it in my AC unit
This looks like it could either be a thermocouple or thermistor. The copper sheathing exists to protect the sensor inside. Many thermistors use a glass-covered sensor which can be easily damaged. A thermocouple generates (mVs of) voltage and a thermistor changes its resistance. If you have a multimeter, measuring the resistance across its wires can determine what it is. A thermistor will read a certain resistance, depending on the temperature. Common values include ~10K and ~100K Ω, but it may vary. A thermocouple will give anything else. Many multimeters have a thermocouple mode, but only for specific types, and probing the sensor is not very easy. Posting a picture of the connector may help identify the sensor, as many thermocouples use standard connectors.

However, as Dmitri and Nick mention in the comments, a thermocouple is unlikely due to noise in motors interfering and the lack of color-coded wires.