The ninth letter of the Greek alphabet is 'IOTA' (4 letters). IOTA is a letter in the Greek alphabet that is used to represent the sound /i/. It is the smallest letter in the Greek alphabet and is often used to represent a small quantity or amount. In mathematics, IOTA is used to denote an infinitesimal quantity. In physics, it is used to denote a unit of electrical current. In linguistics, it is used to denote a short vowel sound.