- A nonmetallic element constituting 21 percent of the atmosphere by volume that occurs as a diatomic gas, O<sub>2</sub>, 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, O<sub>3</sub>, 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. <i>cross-reference</i>: <strong> Periodic Table</strong>.
- Chemical symbol, O; atomic weight, 16. An element discovered by Priestley in 1774, who called it <em>dephlogisticated air.</em>
- A manufacturers' name for bleaching-powder.
- 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.
- Chlorine used in bleaching.
- A chemical <xref>element</xref> (symbol O) with an <xref>atomic number</xref> of 8 and <xref>relative atomic mass</xref> of 15.9994.
- Molecular oxygen (O2), a <xref>colorless</xref>, <xref>odorless</xref> gas at room temperature.
- A mixture of oxygen and other gases, administered to a patient to help him or her to <xref>breathe</xref>.
- An atom of this element.
- 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