- a stable isotope of hydrogen with a mass approximately twice that of the usual isotope. (Symbol: D)
Deuterium atoms have a neutron as well as a proton in the nucleus, and the isotope is present to about 1 part in 6,000 in naturally occurring hydrogen. It is used as a fuel in thermonuclear bombs, and heavy water (D2O) is used as a moderator in nuclear reactors
1930s: modern Latin, from Greek deuteros 'second'