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A gas having molecules consisting of two oxygen atoms, chemical formula O2. Oxygen is an important component of air. This gas is produced by plants during photosynthesis? and is necessary for animals' respiration?.
The process by which oxygen reacts with other elements or chemical compunds is called oxidation, though the term has since been generalised by chemists to also refer to electrochemistry processes not involving oxygen.
See also combustion, Oxidation.
Oxygen is not necessary for oxidation the way chemists use the term (removal of electrons from something). I'm not quite sure how to change the sentence, though, so that it still makes sense.