Definition of a indefinite article from the Oxford Advanced American Dictionary

      

    a

     indefinite article
    indefinite article
    NAmE//ə//
     
    , NAmE////
     
    (also an
    NAmE//ən//
     
    , NAmE//æn//
     
    )
     
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  1. 1 The form a is used before consonant sounds and the form an before vowel sounds. When saying abbreviations like “FM” or “UN,” use a or an according to how the first letter is said. For example, F is a consonant, but begins with the sound /ɛ/ and so you say:an FM radio. U is a vowel but begins with /y/ and so you say:a UN declaration. used before countable or singular nouns referring to people or things that have not already been mentioned a man/horse/unit an aunt/egg/hour/X-ray I can only carry two at a time. There's a visitor for you. She's a friend of my father's (= one of my father's friends).
  2. 2used before uncountable nouns when these have an adjective in front of them, or phrase following them a good knowledge of French a sadness that won't go away
  3. 3any; every A lion is a dangerous animal.
  4. 4used to show that someone or something is a member of a group or profession Their new car's a BMW. She's a Buddhist. He's a teacher. Is that a Monet (= a painting by Monet)?
  5. 5used in front of two nouns that are seen as a single unit a knife and fork
  6. 6used instead of one before some numbers A thousand people were there.
  7. 7 used when talking about prices, quantities, and rates synonym per They cost 50 cents a pound. I can type 50 words a minute. He was driving at 50 miles an hour.
  8. 8 a person like someone He's a little Shaq.
  9. 9 used before someone's name to show that the speaker does not know the person There's a Mrs. Green here to see you.
  10. 10used before the names of days of the week to talk about one particular day She died on a Tuesday.
See the Oxford Advanced Learner's Dictionary entry: a