Zanzibar [a] is an insular semi-autonomous region which united with Tanganyika in 1964 to form the United Republic of Tanzania.It is an archipelago in the Indian Ocean, 25–50 km (16–31 mi) off the coast of the African mainland, and consists of many small islands and two large ones: Unguja (the main island, referred to informally as Zanzibar) and Pemba Island.