(anc. Aesis), a town and episcopal see of the Marches, Italy, in the province of Ancona, from which it is 17 m. W. by S. by rail, 318 ft. above sea-level. Pop. (1901), 23,285. The place took its ancient name from the river Aesis (mod. Esino), upon the left bank of which it lies. It still retains its picturesque medieval town walls. The Palazzo del Comune is a fine, simple, early Renaissance building (1487-1503) by Francesco di Giorgio Martini; the walls are of brick and the window and door-frames of stone, with severely restrained ornamentation. The courtyard with its loggie was built by Andrea Sansovino in 1519. The library contains some good pictures by Lorenzo Lotto. The castle was built by Baccio Pontelli (1488), designer of the castle at Ostia (1483-1486). Jesi was the birthplace of the emperor Frederic II. (1194), and also of the musical composer, Giovanni Battista Pergolesi (1710-1736). The river Aesis formed the boundary of Italy proper from about 250 B.C. to the time of Sulla (c. 82 B.C.); and, in Augustus' division of Italy, that between Umbria (the 6th region) and Picenum (the 5th). The town itself was a colony, of little importance, except, apparently, as a recruiting ground for the Roman army.