n., pl., o's, or O's, also os or Os.
- The 15th letter of the modern English alphabet.
- Any of the speech sounds represented by the letter o.
- The 15th in a series.
- Something shaped like the letter O.
- O One of the four major blood groups in the ABO system. Individuals with this blood group have neither A nor B antigens on the surface of their red blood cells, and have both anti-A and anti-B antibodies in their blood serum.
- A zero.
The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.
