definition of
O

[1] #1 O n. [oxygen, atomic number 8] 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. wn

[2] #2 O n. [o] the 15th letter of the Roman alphabet. wn

[3] #3 O n. [o] the 15th letter of the Roman alphabet. wn

[4] #4 O n. [type O, group O] the blood group whose red cells carry neither the A nor B antigens; "people with type O blood are universal donors". wn