The 9th Greek letter is IOTA, which is represented by the 4 letters 'IOTA'. IOTA is the smallest letter in the Greek alphabet and is used to represent a variety of sounds. It is also used as a symbol for the number 10 in mathematics. In Ancient Greek, IOTA was used to represent the vowel sound 'i' and was often used in the middle of words. In modern Greek, it is used to represent the sound 'ee'. IOTA is also used in various alphabets, such as the Cyrillic and Coptic alphabets.