Definition of A noun from the Oxford Advanced American Dictionary

     

    A

     noun
    noun
    NAmE////
     
    (pl. As, A's
    NAmE//eɪz//
     
    )
     
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  1. 1 a (a's) [countable, uncountable] the first letter of the English alphabet “Apple” begins with (an) A/“A.”
  2. 2 [countable, uncountable] (music) the 6th note in the scale of C major
  3. 3 [countable, uncountable] the highest grade that a student can get for a course, test, or piece of work She got an A inBiology. He had straight A's (= nothing but A's) all through high school.
  4. 4[uncountable] used to represent the first of two or more possibilities Shall we go for plan A or plan B?
  5. 5[uncountable] used to represent a person, for example in an imagined situation or to hide their identity Assume A knows B is guilty. see also A-frame
  6. Idioms
    from A to B
     
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    from one place to another For me a car is just a means of getting from A to B.
    from A to Z
     
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    including everything there is to know about something He knew his subject from A to Z.
See the Oxford Advanced Learner's Dictionary entry: A