o

(noun) a nonmetallic bivalent element that is normally a colorless odorless tasteless nonflammable diatomic gas; constitutes 21 percent of the atmosphere by volume; the most abundant element in the earth's crust. Synonyms: oxygen, O, atomic number 8.

(noun) the 15th letter of the Roman alphabet. Synonyms: O.

(noun) the blood group whose red cells carry neither the A nor B antigens. Synonyms: O, type O, group O. Example: "People with type o blood are universal donors."