an island off the coast of East Africa, part of Tanzania; pop. 1,207, 500 (est. 2009, with Pemba). Under Arab rule from the 17th century, Zanzibar was a prosperous trading port. It became a British protectorate in 1890 and an independent Commonwealth state in 1963, but in the following year the sultan was overthrown and the country became a republic, uniting with Tanganyika to form Tanzania.