An Adjective is a word that is added to a noun or a pronoun.

Exception

Such a word is not called an adjective if it is a noun or pronoun. It is then called a noun or pronoun used as an adjective. In the sentence, Mary's sister borrowed my new book, the words Mary and my, although adjective adjuncts, are not adjectives; the first is a noun and the second is a pronoun used as an adjective. The word new is an adjective.