The term 'NINTH IN A SERIES' can be solved with the four-letter word 'IOTA'. IOTA is the ninth letter of the Greek alphabet, and is often used to refer to something small or insignificant. It is also used to refer to the smallest unit of a currency, such as the euro. In mathematics, it is used to denote a very small quantity. In literature, it is sometimes used to refer to a minor detail or an insignificant event.