O
     n 1: the blood group whose red cells carry neither the A nor B
          antigens; "people with type O blood are universal
          donors" [syn: {type O}, {group O}]
     2: 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 [syn: {oxygen}, {atomic
        number 8}]
     3: the 15th letter of the Roman alphabet
Source: WordNet® 2.0