cryogen

This term is primarily used in technical, scientific, and industrial registers. It refers specifically to the substance itself rather than the process of cooling. While "refrigerant" is a broader term covering any substance used for cooling, "cryogen" specifically implies the extreme low temperatures associated with cryogenics, often involving liquefied gases like helium or nitrogen.

In professional contexts, the word is used to describe the medium required to maintain superconducting states or to preserve biological samples. It is rarely encountered in casual conversation and is almost exclusively found in physics, chemistry, and aerospace engineering literature.