oxygen

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. 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 iron ore. It combines with most elements, is essential for plant and animal respiration, and is required for nearly all combustion. Atomic number 8; atomic weight 15.9994; melting point -218.4°C; boiling point -183.0°C; gas density at 0°C 1.429 grams per liter; valence 2. See Table at element.

Century Dictionary and Cyclopedia

1. n. Chemical symbol, O; atomic weight, 16. An element discovered by Priestley in 1774, who called it dephlogisticated air. It was finally called oxygen by Lavoisier, because supposed to be present in all acids. Further investigation, however, has proved that this is not the case. Oxygen is a chemical element existing as a permanent gas, colorless, odorless, and tasteless, and somewhat heavier than atmospheric air. It is soluble in water, which at a temperature of 60° F. dissolves of its volume of oxygen. Oxygen combines very readily with most of the elements, and forms oxids with all of them excepting fluorin. The act of combination is so energetic in many cases as to evolve light and heat, the phenomena of combustion. In other cases, as in the tarnishing or rusting of metals and the decay of animal or vegetable substances, oxidation takes place so slowly that, while the result is the same, the heat evolved at one time is not enough to produce luminous effects or even to be sensible. Free or uncombined oxygen is essential to all animal and vegetable life. Animal heat and muscular energy are results of a slow combustion produced in all parts of the system by oxygen carried in the blood from the lungs. In sunlight oxygen is exhaled by growing plants, but a certain quantity is assimilated and is essential to life. Oxygen is the most widely distributed and abundant element in nature; it constitutes about one fifth of the total volume of the atmosphere, which is a mechanical mixture of oxygen and nitrogen. Water contains about 89 per cent. of it by weight, and it is found in most animal and vegetable products, acids, oxids, and salts. The rocks which make up most of the earth's crust contain between 40 and 50 per cent. of oxygen. Under certain conditions oxygen may be made to pass into an allotropic or condensed form called ozone.
2. n. A manufacturers' name for bleaching-powder.

Wiktionary

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

GNU Webster's 1913

1. n. (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.
2. n. Manufacturing name Chlorine used in bleaching.

WordNet 3.0

1. n. 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