Tokyo, officially Tokyo Metropolis, is one of the 47 prefectures of Japan. It has served as the Japanese capital since 1869. As of 2018, the Greater Tokyo Area ranked as the most populous metropolitan area in the world. The urban area houses the seat of the Emperor of Japan, of the Japanese government and of the National Diet. Tokyo forms part of the Kantō region on the southeastern side of Japan’s main island, Honshu, and includes the Izu Islands and Ogasawara Islands. Tokyo was formerly named Edo when Shōgun Tokugawa Ieyasu made the city his headquarters in 1603. It became the capital after Emperor Meiji moved his seat to the city from Kyoto in 1868; at that time Edo was renamed Tokyo. The Tokyo Metropolis formed in 1943 from the merger of the former Tokyo Prefecture and the city of Tokyo. Tokyo is often referred to as a city but is officially known and governed as a “metropolitan prefecture”, which differs from and combines elements of a city and a prefecture, a characteristic unique to Tokyo.