1. A grammatical article used to refer to a noun in a non-specific or general way, introducing something for the first time or referring to any member of a class
In English, 'a' and 'an' are indefinite articles.
Em inglês, 'a' e 'an' são artigos indefinidos.
2. The article form that does not specify a particular person or thing, contrasting with definite articles
She bought a book yesterday.
Ela comprou um livro ontem.