Tokyo and its surroundings are often classified as the world's largest metropolitan area. The population of what is known as the Greater Tokyo Area today (2019) amounts to about 38 million people sharing on an area about the size of Los Angeles County in the US or Skåne County in Sweden. What is really included when talking about “Tokyo”, however, can vary dramatically depending on who you ask.
Worth remembering though is that the Greater Tokyo Area almost always includes several other cities as well. For example, Yokohama, the second-largest city in Japan, is also a part of "Greater Tokyo". Kawasaki, Saitama and Chiba, are three other cities with between 1-7 million inhabitants each, are also included.
So why are these other cities also included when talking about Tokyo? Well, the simple fact is that what was once clearly defined cities now have grown together into a single megacity, with hardly any empty space between them to speak of. When you travel on a train from Tokyo station to Sakuragicho in Yokohama, Kawaguchi in Saitama or Funabashi in Chiba, it's almost impossible to distinguish where one city ends and another starts. In fact, the Kanto Plain, where Tokyo and all its neighbouring cities are located, is one of the most densely populated areas in the world!
But back to Tokyo. Another concept that any visitor should be familiar with is "the 23 special wards of Tokyo". These constitute a much smaller geographical area, and what one could call "Central Tokyo". Administration of a city as large as Tokyo is challenging to adapt to references that someone from less populated countries can even fathom. Things in Japan are simply organized on so many layers that it might be challenging to wrap one's head around it, and on to of that, it's also easy to get confused by the terminology. But I will make an effort flesh it out now.
So what is a "ward"? Well, think of Brooklyn, Manhattan and the Bronx in New York or Kreuzberg and Mitte in Berlin, and you are on the right track. Oddly enough, the official translation of the ward names, which all end with -ku in Japanese, for example, Minato-ku, Shibuya-ku and Shinjuku-ku, is to give them the suffix "-city". Although the usual meaning of the word "city" is quite different from what these special wards constitute. Each Ward has its own responsibilities. Things such as taxes, garbage collection, local-level infrastructure projects, construction permits etc. are all handled on this level. These 23 special districts make up the most densely populated part of the Tokyo urban area. In total, they have about 9 million inhabitants.
But the wards are just a part of the bigger puzzle. Another, perhaps more important definition, is Tokyo-to, which in English goes by the name "Tokyo Metropolis". In addition to the 23 wards, Tokyo also consists of 26 "shi" (cities in English), five "Cho" (towns in English) and eight "mura" (villages). All these are included in Tokyo Metropolis. As you can see in the picture above, the city extends from east to west, with the central parts (the 23 special districts in purple) to the east. The western parts are increasingly mountainous and the population density drops as you travel further into the mountainous regions. In total, Tokyo-to has about 13.5 million inhabitants.
Tokyo-to also includes a bunch of islands called "Tokyo-shoto" in Japanese. These stretch out in the Pacific, starting in the Gulf of Tokyo and continuing to the south. The biggest of these islands is Izu-Oshima, located about 22 km off the coast of the Izu Peninsula, which is the nearest point on the Japanese mainland. Izu Oshima has a population of about 8000 people and is a great weekend getaway from Tokyo.
At the far end of the chain of islands lies the Ogasawara Archipelago, where about 3000 people live. Almost all of these are based on the main island of Chichijima ("Daddy Island"). To get to the Ogasawara Islands, a boat trip of about 30 hours is required - no flight connections are available.
Although life on these tropical South Island islands differs dramatically from that in the big city, the residents are still formally residents of the city of Tokyo, or Tokyo Metropolis, talk about it being a city of contrasts!
In fact, Chichijima is one of my dream destinations in Japan, the island ended up on my radar after reading about this fancy place (http://www.spoon-tamago.com/2016/02/04/pat-inn-a -modernist-hotel-on-Tokyo-tropical-Chichijima /). If all goes well, maybe I can come back with a report of how life looks in Tokyo's most isolated neighbourhoods here on this website soon.
If you want to dig deeper into this subject and learn almost everything there is to know about the many ways to define the greater Tokyo Area, I highly recommend taking a look at this Wikipedia article: https://en.wikipedia.org/wiki/Greater_Tokyo_Area.