oxygen, O, atomic number 8 - a nonmetallic bivalent element that is normally a colorless odorless tasteless nonflammable diatomic gas; constitutes 21 percent of the atmosphere by volume; the most abundant element in the earth's crust --1is a kind ofchemical element, element; gas --1is a substance of water, H2O; air; ozone --1has particulars: liquid oxygen, LOX