From :

Deuterium is the isotope of hydrogen with one proton and one neutron, D = 2H. On average, one out of every 6700 hydrogen atoms found in nature is deuterium. Processes are well developed for separating water molecules containing deuterium atoms, or "heavy water," so that the deuterium fuel for fusion reactions is essentially free.

Also a popular name for computers with 2 CPUs.

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