An Article is used before a noun to indicate if the noun is something particular (the) or a member of a class (a/an).
Article is the term used for a modifying word that introduce a noun phrase, and that implies that:
- The thing referred to is not specific (e.g. she bought me a book)
- The thing referred to has already been mentioned (e.g. she bought me the book I wanted to buy last summer)
In the first case, the article is the indefinite article; in the second case, the article is the definite article.