(anciently Pontia), the largest and only inhabited island of a small group in the Mediterranean, lying off the coast of Naples, included in the province of Terra di Lavoro, and 29 miles S.W. of Terracina. It is crescent-shaped, 5 miles long and 1 or 2 broad, and has a natural harbour on the concave side, which looks towards the mainland. Near the harbour stand a castle, governor's house, storehouses, and a few houses and huts. The surface is entirely rocky, and generally barren; but there are some good pastures for cattle. The inhabitants are chiefly employed in fishery and salt-works. Pop. 1500. The other islands of the group are Palmarola and Zannone. Ponza and Palmarola were used as places of banishment by the Romans.