The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   Note: Although a gas, hydrogen is chemically similar to the
         metals in its nature, having the properties of a weak
         base. It is, in all acids, the base which is replaced
         by metals and basic radicals to form salts. Like all
         other gases, it is condensed by great cold and pressure
         to a liquid which freezes and solidifies by its own
         evaporation. It is absorbed in large quantities by
         certain metals (esp. palladium), forming alloy-like
         compounds; hence, in view of quasi-metallic nature, it
         is sometimes called hydrogenium. It is the typical
         reducing agent, as opposed to oxidizers, as oxygen,
         chlorine, etc.
         [1913 Webster]

   Bicarbureted hydrogen, an old name for ethylene.

   Carbureted hydrogen gas. See under Carbureted.

   Hydrogen dioxide, a thick, colorless liquid, H2O2,
      resembling water, but having a bitter, sour taste,
      produced by the action of acids on barium peroxide. It
      decomposes into water and oxygen, and is manufactured in
      large quantities for an oxidizing and bleaching agent.
      Called also oxygenated water.

   Hydrogen oxide, a chemical name for water, H?O.

   Hydrogen sulphide, a colorless inflammable gas, H2S,
      having the characteristic odor of bad eggs, and found in
      many mineral springs. It is produced by the action of
      acids on metallic sulphides, and is an important chemical
      reagent. Called also sulphureted hydrogen.
      [1913 Webster]

WordNet (r) 3.0 (2006):

hydrogen
    n 1: 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: hydrogen, H, atomic number 1]

The Elements (07Nov00):

hydrogen
Symbol: H
Atomic number: 1
Atomic weight: 1.0079
Colourless, odourless gaseous chemical element. Lightest and most
abundant element in the universe. Present in water and in all organic
compounds. Chemically reacts with most elements. Discovered by Henry
Cavendish in 1776.