Radiator coolant is a liquid that helps regulate the temperature of a car's engine. It flows through the radiator to cool the engine down, preventing it from overheating. Full definition
I have replace thermostat valve and now I can see engine oil mix with radiator coolant. (mechanics.stackexchange.com)
If it is, this would be indicative of a compromised head gasket, warped cylinder head, or damaged oil cooler (if the car uses radiator coolant for cooling the oil). (mechanics.stackexchange.com)