a pronoun is usually defined as a word that takes the place of a noun. However, they point out that certain pronouns, such as none, nobody, anything and impersonal it, do not take the place of any noun but are words more or less arbitrarily classified by grammarians and lexicographers as pronouns.
Another definition of pronoun is: a pronoun is a word that is used to replace a noun or a noun group that has already been mentioned or that will be mentioned later