The term 'informal affirmatives' refers to informal words or phrases that are used to express agreement or acceptance. The most common informal affirmative is 'yeahs, ' which is a five-letter word. It is an informal way of saying 'YES' and is often used in casual conversations. Other informal affirmatives include 'YUP, ' 'uh-huh, ' and 'SURE.' These words are used to show agreement or acceptance without being too formal.