plural A's, a's
the first letter of the English alphabet
also an, strong indefinite article, determiner
1 used before a noun that names something or someone that has not been mentioned before, or that the person you are talking to does not know about: Do you have a car? | There's a spider in the bath.-compare the 12a) used before a noun that is one of a particular group or class of people or things: I want to train to be a teacher.b) used before someone's family name to show that they belong to that family: Only a Peterson would drive a car like that!3a) one: a thousand pounds | a dozen eggsb) a lot/a few/a little/a great deal etc used before certain words that express an amount of something: There were a lot of people at the party. | A few weeks from now I'll be in Venice.4 twice a week/$5 a day etc two times each week, $5 each day etc; per: I get paid once a month. | The eggs cost $2 a dozen.5 used before a noun to mean all things of that type: A square has four sides. (=all squares have four sides)6 used before two nouns that are mentioned together so often that they are thought of as one thing: a cup and saucer | Does everyone have a knife and fork?7a) used before singular nouns, especially words for actions, meaning one example of that action: Take a look at this. | It needs a good clean.b) used before the -ing form of verbs when they are used as nouns: a crashing of gearsc) used before an uncountable noun when other information about the noun is added by an adjective or phrase: Candidates must have a good knowledge of chemistry. | a beauty that became legendary8 used before an uncountable noun to mean a type of it: a particularly fine Stilton cheese9 used before the name of a painter or artist etc meaning a particular painting, sculpture etc by that person: an early Rembrandt10 used before a name to mean having the same qualities as that person or thing: She was hailed as a new Marilyn Monroe.11a) used before someone's name when you do not know who they are: There is a Mr Tom Wilkins on the phone for you. | a certain: A certain Lisa Blair wishes to speak to you.b) used before names of days, events in the year etc to mean a particular one: I can't remember a Christmas like it.12 used after such, what, rather and (formal) many to emphasize what you are saying: What a day! I was late for work and my car broke down. | She had spent many a night (=many nights) waiting for him to come home.III. noun1 also a (C, U) the sixth note in the musical scale 1 (8) of C major or the musical key 1 (4) based on this note2 (C) the highest mark that a student can get in an examination or for a piece of work: I got an A in French.3 an A student AmE someone who regularly gets the best marks possible for their work in school or college4 from A to B from one place to another: Hiring a car was the best way to get us from A to B.5 from A to Z describing, including, or knowing everything about a subject: the history of 20th century art from A to Z6 A3/A4 standard sizes of paper in the European Union7 (U) a common type of bloodIV. the written abbreviation of amp V. prefix1 in a particular condition or way: alive (=living) | aloud | with nerves all atingle (=tingling)2 old use in, to, at, or on something: abed (=in bed) | afar (=far away)VI. prefix showing an opposite or the absence of something; not; without: amoral (=not moral) | atypically (=not typically) VII. adjective old-fashioned very good or completely healthy: Everything about the resort was A-1.
Longman dictionary of contemporary English. 2004.