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. As of 2014 the Greater Tokyo Area ranked as the most populous metropolitan area in the world.
Tokyo was originally known as Edo, which means "estuary". Its name was changed to Tokyo when it became the imperial capital with the arrival of Emperor Meiji in 1868, in line with the East Asian tradition of including the word capital in the name of the capital city (like Kyoto, Beijing and Nanjing). During the early Meiji period, the city was also called "Tokei", an alternative pronunciation for the same characters representing "Tokyo", making it a kanji homograph.