Niihau (nēˈhou) [key], island (1990 pop. 230), 70 sq mi (181 sq km), in Kauai co., Hawaii, W of Kauai island. It is mostly arid lowland, rising to 1,281 ft (390 m) at Paniau Mt. The island is suitable for cattle grazing. Niihau, privately owned since 1864 when it was purchased from the Hawaiian kingdom by an American family, is operated as a ranch.