(1) A gaseous element, atomic no. 8, atomic wt. 15.9994 on the basis of 12C = 12.0000; an abundant and widely distributed chemical element, which combines with most other elements to form oxides and is essential to animal and plant life. (2) The molecular form of oxygen, O2, also called dioxygen. (3) A medicinal gas that contains not less than 99.0%, by volume, of O2. [G. oxys, sharp, acid and genes, forming]