o

noun

1. 1

   the blood group whose red cells carry neither the A nor B antigens

   people with type O blood are universal donors

2. 2

   the 15th letter of the Roman alphabet

   people with type O blood are universal donors

3. 3

   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