The definite article is the word "the." It is used to refer to a specific noun that is already known to the reader or listener. For example, if you say "the book," it implies that both you and the listener know which book you are talking about, rather than any book in general. Using "the" helps to clarify communication. For instance, if you mention "the teacher," it indicates a particular teacher, perhaps one that has been previously discussed. This specificity makes it easier for people to understand exactly what you mean, avoiding confusion with other possible nouns.