1. (Chem.) A colorless, tasteless, odorless, gaseous element 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 15.96. Note: It occurs combined in immense quantities, forming eight ninths by weight of water, and probably one half by weight of the entire solid cr...