an Definition

1. Used chiefly with a singular noun, usually where the thing referred to has not been mentioned before, or where it is not a specific example known to the speaker or listener. Compare the.
2. Used before a word describing quantity.

Example: a dozen eggs
3a. Any or every;

Example: A fire is hot
3b. Used after not or never: any at all.

Example: not a chance
4. Each or every; per.

Example: once a day
5. One of a specified type.

Example: He thinks he's a real Romeo

Some people use an before words beginning with a weakly sounded h, eg an hotel, an historic occasion. This use of an is no more nor less correct than a; however, it is sometimes regarded as old-fashioned.

See Usage Note at a1.