oxygen

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

• noun A nonmetallic element constituting 21 percent of the atmosphere by volume that occurs as a diatomic gas, O₂, and in many compounds such as water and silica, and in iron ore. It combines with most elements, is essential for plant and animal respiration, and is required for nearly all combustion. Ozone, O₃, is an allotrope of this element. Atomic number 8; atomic weight 15.9994; melting point −218.79°C; boiling point −182.9°C; gas density at 0°C 1.429 grams per liter; valence 2. cross-reference: Periodic Table.

from The Century Dictionary.

• noun Chemical symbol, O; atomic weight, 16. An element discovered by Priestley in 1774, who called it dephlogisticated air.
• noun A manufacturers' name for bleaching-powder.

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

• noun (Chem.) A colorless, tasteless, odorless, gaseous element of atomic number 8, occurring in the free state in the atmosphere, of which it forms about 23 per cent by weight and about 21 per cent by volume, being slightly heavier than nitrogen. Symbol O. Atomic weight 15.9994.
• noun Manufacturing name Chlorine used in bleaching.

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

• noun A chemical element (symbol O) with an atomic number of 8 and relative atomic mass of 15.9994.
• noun Molecular oxygen (O2), a colorless, odorless gas at room temperature.
• noun medicine A mixture of oxygen and other gases, administered to a patient to help him or her to breathe.
• noun countable An atom of this element.

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

• noun 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