A nonmetallic element, normally colorless, odorless, tasteless, nonflammable diatomic gas. Symbol, O. Occurs uncombined in the air to the extent of about 21% by volume and is combined in water, in most rocks and minerals, and in a great variety of organic compounds. Oxygen is very reactive and capable of combining with most elements. Essential for respiration in all plants and animals and for practically all combustion. Oxygen enrichment of steel blast furnaces accounts for the greatest use of the gas. Used in manufacturing ammonia, methanol, and ethylene oxide.