O

noun

1. 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
2. the 15th letter of the Roman alphabet
3. the 15th letter of the Roman alphabet
4. the blood group whose red cells carry neither the A nor B antigens"people with type O blood are universal donors"