letter a

1 . ( the indefinite article ) used when referring to someone or something for the first time in a text or conversation.
2. used with units of measurement to mean one such unit.
3. one single; any.
4. used to indicate membership of a class of people or things.
5. used when expressing rates or ratios; in, to, or for each; per.
6. the first letter of the alphabet.
7. denoting the first in a set of items, categories, sizes, etc.
8. denoting the first of two or more hypothetical people or things.
9. the highest class of academic mark.
10. (A) the human blood type (in the ABO system) containing the A agglutinogen and lacking the B.