Is the called an indefinite article

In English there are three articles: a, an, and the. The definite article (the) is used before a noun to indicate that the identity of the noun is known to the reader. The indefinite article (a, an) is used before a noun that is general or when its identity is not known

lab rats that ate more artifical sweeter gained more weight than those who ate sugar and they had “lower levels of a hormone called glp-1, which affects how full we feel.”