Definition of a indefinite article from the Oxford Advanced Learner's Dictionary

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a

indefinite article
ə
 
; ə
 
 
; strong form
 
(also an)
 
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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 /e/ and so you say: an FM radio. U is a vowel but begins with /j/ and so you say: a UN declaration.
1 used before countable or singular nouns referring to people or things that have not already been mentioneda man/horse/unitan aunt/egg/hour/x-rayI 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 used before uncountable nouns when these have an adjective in front of them, or phrase following thema good knowledge of Frencha sadness that won't go away3 any; everyA lion is a dangerous animal.4 used to show that somebody/something is a member of a group or professionTheir new car's a BMW.She's a Buddhist.He's a teacher.Is that a Monet (= a painting by Monet)?5 used in front of two nouns that are seen as a single unita knife and fork6 used instead of one before some numbersA thousand people were there.7 used when talking about prices, quantities and rates
Synonym
per
They cost 50p a kilo.I can type 50 words a minute.He was driving at 50 miles an hour.
8 a person like somebodyShe's a little Hitler.9 used before somebody's name to show that the speaker does not know the personThere's a Mrs Green to see you.10 used before the names of days of the week to talk about one particular dayShe died on a Tuesday.