an island off the N. coast of Sicily, 412 m. N.N.W. of Palermo. Pop. (1861) 2231; (1901) 1916. It is the Osteodes of the Greeks, but in Roman times was known as Ustica. The island is entirely volcanic and subject to earthquakes, and is fertile. There is a considerable penal colony. There are some Roman tombs excavated in the rock.