Terceira Islan (pron. IPA: [tɨɾ'sɐiɾɐ], Portuguese: Ilha Terceira) is an island in the Azores, in the middle of the North Atlantic Ocean, with an area of 396.75 km². The island's length is 29 km and the width is 18km; the perimeter is 90 km. Population is 54,996, down from a peak of 59,000. Population density is 140.73/km².