o meaning and definition

O noun

Definition of 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
   • synonyms: atomic number 8, oxygen
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"
   • synonyms: group O, type O