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See also: Oxygen


Chemical element
O Previous: nitrogen (N)
Next: fluorine (F)


Borrowing from French oxygène (originally in the form principe oxygène, a variant of principe oxigine ‘acidifying principle’, suggested by Lavoisier), from Ancient Greek ὀξύς (oxús, sharp) + γένος (génos, birth), referring to oxygen's role in the formation of acids.



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oxygen (countable and uncountable, plural oxygens)

  1. A chemical element (symbol O) with an atomic number of 8 and relative atomic mass of 15.9994.
  2. Molecular oxygen (O2), a colorless, odorless gas at room temperature.
    • 2013 September-October, Katie L. Burke, “In the News”, in American Scientist:
      Oxygen levels on Earth skyrocketed 2.4 billion years ago, when cyanobacteria evolved photosynthesis: the ability to convert water and carbon dioxide into carbohydrates and waste oxygen using solar energy. The evolutionary precursor of photosynthesis is still under debate, and a new study sheds light.
  3. (medicine) A mixture of oxygen and other gases, administered to a patient to help him or her to breathe.
  4. (countable) An atom of this element.
    • 2013, Spencer L. Seager, ‎Michael R. Slabaugh, Chemistry for Today: General, Organic, and Biochemistry (page 479)
      Look first at any structure to see if there is a carbon with two oxygens attached. Hemiacetals, hemiketals, acetals, and ketals are all alike in that regard.

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oxygen n (singular definite oxygenet, not used in plural form)

  1. oxygen