former Islāmic city-state on an island off the coast of what is now southern Tanzania. Founded in the late 10th century by settlers from Arabia and Iran, it became one of the most active commercial centres on the east coast of Africa. Held briefly by the Portuguese (1505–12), it thereafter gradually declined in importance and was finally abandoned. Extensive ruins remain, including mosques, a Portuguese fort, and the great Husuni Kubwa palace (13th–14th century).