SHIN shĭn (שׁ). The twenty-first letter of the Heb. alphabet. It is found at the head of the twenty-first section of Psalm 119, in which section each verse begins with this letter. It also came to be used for the number 300.
International Standard Bible Encyclopedia (1915)
shen, sen "sh", "s": The 21st letter of the Hebrew alphabet; transliterated in this Encyclopedia as "sh" and "s". It came also to be used for the number 300. For name, etc., see Alphabet.