deuterium

noun

Etymology: From deutero- + -ium. Coined by American physical chemist Harold Urey, from Ancient Greek δεύτερος (deúteros, “second”).

1. An isotope of hydrogen with one proton and one neutron in each atom - 21H.

   “Heavy water is "heavy" because it contains deuterium.”

2. An atom of this isotope.

   “There were about 80 deuteriums for every million protiums, and virtually no tritium.”