Source : Webster's Revised Unabridged Dictionary (1913)

Hydrogen \Hy"dro*gen\, n. [Hydro-, 1 + -gen: cf. F.
   hydrog[`e]ne. So called because water is generated by its
   combustion. See {Hydra}.] (Chem.)
   A gaseous element, colorless, tasteless, and odorless, the
   lightest known substance, being fourteen and a half times
   lighter than air (hence its use in filling balloons), and
   over eleven thousand times lighter than water. It is very
   abundant, being an ingredient of water and of many other
   substances, especially those of animal or vegetable origin.
   It may by produced in many ways, but is chiefly obtained by
   the action of acids (as sulphuric) on metals, as zinc, iron,
   etc. It is very inflammable, and is an ingredient of coal gas
   and water gas. It is standard of chemical equivalents or
   combining weights, and also of valence, being the typical
   monad. Symbol H. Atomic weight 1.

Source : WordNet®

hydrogen
     n : a nonmetallic univalent element that is normally a colorless
         and odorless highly flammable diatomic gas; the simplest
         and lightest and most abundant element in the universe
         [syn: {H}, {atomic number 1}]