hydrogen

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A colorless, highly flammable element, that occurs as a diatomic molecule, H₂, the lightest of all gases and the most abundant element in the universe, used in the production of synthetic ammonia and methanol, in petroleum refining, in the hydrogenation of organic materials, as a reducing atmosphere, in oxyhydrogen torches, in cryogenic research, and in rocket fuels. Atomic number 1; atomic weight 1.00794; melting point −259.1°C; boiling point −252.8°C; density at 0°C 0.08988 gram per liter; valence 1. cross-reference: Periodic Table.

from The Century Dictionary.

• noun Chemical symbol, H. One of the elementary substances, existing as a colorless, tasteless, and inodorous gas.
• noun H₂S, a colorless inflammable gas having a sweetish taste and an exceedingly fetid smell resembling rotten eggs. It is extremely poisonous when inhaled. It has feeble acid properties, and its compounds with bases are called sulphids. It occurs in the emanations of volcanoes, and is evolved when animal or vegetable tissue containing sulphur decays. It also occurs in mineral springs, being liberated by the reduction of gypsum or other sulphates through the action of a microbe.
• noun Hydrogen compounds with strongly electronegative elements or radicals, easily exchanging hydrogen for strongly electropositive elements or radicals to form salts, are the same as acids: as hydrogen chlorid (hydrochloric acid), hydrogen sulphate (sulphuric acid), etc.

from the GNU version of the Collaborative International Dictionary of English.

• noun (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.
• noun an old name for ethylene.
• noun See under Carbureted.
• noun 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.
• noun a chemical name for water, H�O.
• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The lightest chemical element (symbol H) with an atomic number of 1 and atomic weight of 1.00794.
• noun Molecular hydrogen (H2), a colourless, odourless and flammable gas at room temperature.
• noun An atom of the element.
• noun A sample of the element.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun 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