deuterium

This term is used exclusively within scientific and technical registers, specifically in chemistry, nuclear physics, and energy research. It describes a specific isotope of hydrogen, distinguishing it from protium (the most common isotope) and tritium (the radioactive isotope).

As a chemical element isotope, it is treated as an uncountable mass noun. It refers to the substance itself rather than individual atoms in standard usage, meaning it does not take a plural form in professional scientific discourse.