An Article is a word placed before a noun to show whether the latter is used in a particular or general sense.
There are two articles, a or an and the. A or an is called the indefinite article because it does not point out any particular person or thing but indicates the noun in its widest sense; thus, a man means any man whatsoever of the species or race.
The is called the definite article because it points out some particular person or thing; thus, the man means some particular individuals.
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