Refers to the operating fluid that serves as the refrigeration cycle. Inside a freezer, a refrigerant serves as a heat carrier and recirculates, but is not consumed. Therefore, the refrigerant must be a stable compound that does not decompose even if the refrigeration cycle of evaporation, compression, condensation, and expansion is repeated, evaporates and condenses at an appropriate temperature and pressure, is not corrosive to metals, and is harmless to humans and animals even if leaked.