1) An element with atomic symbol O, atomic number 8, and atomic weight 16. It is the most abundant element on earth and essential for respiration.

2) A colorless tasteless odorless gaseous element that constitutes 21 percent of the atmosphere and is found in water, in most rocks and minerals, and in numerous organic compounds, that is capable of combining with all elements except the inert gases, that is active in physiological processes, and that is involved especially in combustion processes -- symbol O.