AiM TR (Thermo Resistor) sensor for measuring fluid temps. Is threaded M10x0.8mm for easy installation in many metric applications.
This is the newer style which features a small probe on the end of the sensor for better temperature gathering.
UPDATE: New part number X05TRM10A4512BPRS vs old part number 3CVTRM1003C2K
The more common 1/8-27 NPT sensor can be found here.