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Definition of hydrogen noun from the Oxford Advanced Learner's Dictionary

 

hydrogen

 noun
noun
BrE BrE//ˈhaɪdrədʒən//
 
; NAmE NAmE//ˈhaɪdrədʒən//
 
[uncountable] (symbol H)
 
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a chemical element. Hydrogen is a gas that is the lightest of all the elements. It combines with oxygen to form water. Word Origin late 18th cent.: coined in French from Greek hudro- ‘water’ + -genēs ‘-born, of a specified kind’, from gen- (root of gignomai ‘be born, become’, genos ‘a kind’).
See the Oxford Advanced American Dictionary entry: hydrogen