Definition
An isotope of hydrogen whose nucleus contains one proton and one neutron, giving it roughly twice the mass of ordinary hydrogen. Deuterium occurs naturally in small amounts and is used in certain nuclear and scientific applications.
Plain English
A heavier form of hydrogen. Regular hydrogen has just one particle in its nucleus; deuterium has two, so each atom weighs about twice as much.
Context Anchor
Seen in chemistry, fuel, materials, or nuclear-energy discussions; it is not a normal day-to-day cockpit term.
Derivation
From the Greek 'deuteros' meaning 'second.' It was named the 'second' form of hydrogen because it is the second-lightest hydrogen isotope after ordinary hydrogen (protium).
Why Pilots Care
A pilot will rarely use this term in normal flying, but recognizing it as a form of hydrogen prevents confusing it with a separate aviation gas, fuel, or aircraft system.
Intuition Check
Deuterium is not a different element from hydrogen. It is hydrogen with a heavier atom.
Example Sentence 1
Deuterium is sometimes called heavy hydrogen because each atom carries an extra neutron.
Example Sentence 2
Technicians checked the deuterium tracer used during the engine test run.