The term 'Agreement Word' refers to words that are used to signify agreement or acceptance. The two most common agreement words are 'AYE' and 'OKAY'. AYE is a three-letter word that is used to express assent, especially in a vote. It is derived from the Old English word 'gea', which means 'YES'. OKAY is a four-letter word that is used to indicate agreement, approval, or acceptance. It is derived from the mid-19th century American slang phrase 'oll korrect', which was used to express agreement.