A liquid or gas used to transfer heat, e.g., in a power plant. A heat transfer medium typically circulates in a closed circuit, at one end taking heat from a heat source and at the other end passing that heat on. Typical heat transfer media are fluids like water, oil, molten salts, or gases like helium or carbon dioxide. Their heat capacity, boiling point, and low chemical reactivity with the piping are important. In the case of a heat transfer medium for a nuclear power plant, it is important to know its ability to absorb or moderate neutrons so that the heat transfer medium does not negatively affect the fission reaction.