Si·en·a (sē-ěn'ə, syě'nä) A city of west-central Italy south of Florence. Founded by Etruscans, it became independent in the 12th century and gradually evolved into a wealthy city known especially for its leadership of the Sienese school of art (13th-14th centuries). Population: 53,800. Si'e·nese' (-nēz', -nēs') adj. & n.