Which term describes extremely low refrigerant temperatures?

The term that best describes extremely low refrigerant temperatures is "cryogenics." This field involves the study and application of materials and processes at very low temperatures, typically below -150 degrees Celsius (-238 degrees Fahrenheit). In the context of refrigeration and air conditioning, cryogenics goes beyond ordinary cooling techniques and pertains to the manipulation and behavior of gases and liquids at these extreme temperatures.

While "subzero" might suggest temperatures below freezing, it does not specifically pertain to the scientific principles or applications associated with extremely low temperatures found in cryogenics. "Refrigeration" refers broadly to the technology and methods used for cooling substances, but it doesn’t capture the specific extremity of temperatures. "Thermal dynamics," likely referring to thermodynamics, pertains to the study of heat transfer and energy changes but does not directly define low temperature conditions or refrigerant behaviors.

Therefore, cryogenics is the most appropriate term for describing processes and phenomena that occur at these extremely low refrigerant temperatures.