hy ‧ dro ‧ gen /ˈhaɪdrədʒ ə n/ BrE AmE noun [uncountable]
[ Date: 1700-1800 ; Language: French ; Origin: hydrogène , from hydro- ( ⇨ ↑ hydro- ) + -gène 'producing' ; because it produces water when it is burned ]
a colourless gas that is the lightest of all gases, forms water when it combines with oxygen, and is used to produce ↑ ammonia and other chemicals. It is a chemical ↑ element : symbol H