The Adult is one of the “roles” recognized in Transactional Analysis (link to definition below??), a communication theory developed by psychiatrist Eric Berne. The adult thinks and acts as a mature adult under all circumstances. The adult is rational and calm and does not get “caught up in” the emotions or drama of a situation.
An example would be when a child is screaming because they can’t stay out late the parent speaking as an adult and simply saying, “When you are calm we can discuss what you want.”