O Definition

1. computing.
Input/output.
Example: I/O system

1. The fifteenth letter of the English alphabet.
2. The shape of this letter.

Example: formed his mouth into an O
3. In telephone, etc jargon: zero; nought.

1. Ocean.
2. Octavo.

Form: o (also)
3. See under OH.
4. Old.
5. Ordinary, in eg O level or O Grade.

1. chem.
Oxygen.
2. An ABO blood group system.