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

   • oxygen, O, atomic number 8

   Less specific

   • chemical element, element, gas

   More specific

   • liquid oxygen, LOX

   Made from

   • water, H2O, air, ozone

2. the 15th letter of the Roman alphabet

   Synonyms

   • O

   Less specific

   • letter, letter of the alphabet, alphabetic character

   Member of

   • Roman alphabet, Latin alphabet

3. the blood group whose red cells carry neither the A nor B antigens; “people with type O blood are universal donors”

   Synonyms

   • O, type O, group O

   Less specific

   • blood group, blood type