hydrogen |ˈhīdrəjən|nouna colorless, odorless, highly flammable gas, the chemical element ofatomic number 1. (Symbol: H)Hydrogen is the lightest of the chemical elements and has the simplest atomic structure, a single electron orbiting a nucleus consisting of a single proton. It is by far the commonest element in the universe, although not on the earth, where it occurs chiefly combined with oxygen as water.DERIVATIVEShydrogenous |hīˈdräjənəs| adjectiveORIGIN late 18th cent.: coined in French from Greek hudro- ‘water’ +-genēs (see -gen ).