Mega City

A city whose population is larger than a reasonably sized country. The actual size can depend on the average city size of the setting, but for modern or sci-fi settings, you can assume that the number of citizens goes into the hundred of millions.

Depending on the setting you can expect to see a lot of high houses, endless suburbs and futuristic ways to come around. If you are lucky the author may even thought on the troubles with providing food and water for all the inhabitants as well as the trouble for the environment so many people naturally generate. If you are unlucky however the heroes may find themselves alone at times and places where that should be impossible given the population density.

Notice that despite the introduction a given city does not count as a Mega City just because it is bigger than a given country. Compare to example real life where not less than 41 countries around the world have less than a million inhabitants, 17 even less than 100,000, where cities of that size in other countries are normal and therefore would not be seen as mega cities.

This will probably be the Capital City, and if it's not a Merchant City, there'll definitely be a Bazaar of the Bizarre if you know where to look. If it is on a restricted land area, expect it to be a Skyscraper City. Occasionally, a Mega City will be so large it is effectively a City Planet.

Examples of Mega City include:

Comic Books

The City in Transmetropolitan.
The world of Judge Dredd is divided into these kind of cities.
- Dredd's home city of Mega City One grew from BosWash (See Real Life examples below) until it covered the entire eastern seaboard. In fact, the whole point of the Apocalypse War arc was to trim its sheer size down, as it had become too big.
Downlode in Sinister Dexter stretches from northern Spain to west Poland.
The Mighty Thor #372, part of a storyline involving Time Travel, says that in the future the entire east coast of the USA will be covered by the megacity of Brooklynopolis (which will be policed by the oddly-familiar Justice Peace).

Film

The pre-war human cities in The Matrix, which were enormous and needed armies of robots to maintain.

Trinity: There used to be cities that spanned hundreds of miles.

The Big Applesauce has become this in the 1930's films Metropolis and Just Imagine—in those days New York was pretty much the only skyscraper city in existence.
Avatar: In the extended version, and in supplementary materials the cities of Earth have become this.

Literature

In Isaac Asimov's The Caves of Steel, all of Earth's population lives in eight hundred arcologies known as 'Cities'. The average population of each City is eleven point two million. The governments of three large cities (New York, Philadelphia and Washington) are considering merging into one single Mega Mega City, but the logistics of maintaining and governing such a large conglomerate have so far prevented any action on the plan.
- Later in the Robots/Empire/Foundation series, Trantor, and presumably a few other worlds, are globe-spanning versions of these with populations in the billions.
Discworld: While Ankh-Morpork only has a million inhabitants, it is still the biggest city in the setting. In The Fifth Elephant, Carrot points out that it's actually the largest dwarf city, having more dwarfs than any purely dwarf city. (This isn't as far-fetched as it may seem—a Real Life example is the city of São Paulo, whose Italian community is larger than any city in Italy itself.)
In the Myth Adventures series, the dimension of Deeva is entirely taken up by an open-air market.
The 13½ Lives of Captain Bluebear: The Capital of the continent of Zamonia, Atlantis, has over 200 million inhabitants.
The arcologies in The Night's Dawn Trilogy.
In Vladimir Vasilyev's Big Kiev Technician Urban Fantasy series, Big Kiev in this Alternate Universe is roughly 600 miles in diameter. Big London is also mentioned, although it may be smaller. It takes 9 hours on a train to get from the Black Sea in the southern part of town to the Center (where our Kiev, Ukraine, is located). One of the old cities absorbed into Big Kiev is L'viv. Only canned goods are available in the Center due to lack of farmland or cattle so far from the edge, unless you're willing to spend a fortune on fresh meat and fruit.
- Big Moscow is mentioned to be even larger, although younger.
  - A military conflict is mentioned to have happened in the past between Big Moscow and Big Berlin. Does This Remind You of Anything?
- This can be partially explained by cities apparently growing on their own, as a jungle would. This fits into the overall theme of all machines in this world being alive.
  - Apparently, the major cities have been slowly growing for the past 300,000 years in the novel's timeline.
William Gibson's Sprawl Trilogy has the titular Sprawl, more officially known as BAMA, the Boston-Atlanta Metropolitan Axis, presumably an extension of the real Bos-Wash. Also notable for being almost completely covered by geodesic domes. Judging from descriptions in Neuromancer Chiba City and the other cities around Tokyo Bay also count.
In one of Andrei Livadny's The History of the Galaxy novels, Earth cities are mentioned to have become these with populations of major cities approaching 20 billion each. That's nearly triple the population of the entire world today in a single city. This is the main reason why President John Winston Hammer of the Earth Alliance sends a fleet to force the recently-discovered Lost Colonies into submission, so as to offload the extra population. By the time of the later novels, most of the population has moved to new colonies or died in the First Galactic War (the 30-year war with the colonies, which Earth ultimately loses) with only about 100 million people left on Earth. Most urban areas, now abandoned, are covered by lush jungles, and many surviving landmarks have been moved to other areas for preservation. After the war, the colonies emerge as the industrial, economic, and scientific power in human space, forming the Confederacy of Suns for mutual protection (Earth isn't included).

Tabletop Games

Warhammer 40,000: The Imperial Palace covers all of Eurasia. (The Golden Throne and Astronomicon, counting attendant machinery and security, has a volume equal to the interior of Mount Everest all on its own.) Though what modern continents match up to what building (yes, building) sometimes varies.
- Also, Imperial Hive Cities.
- Beyond the Imperial Palace, Earth (or "Holy Terra" as it is officially designated) has been depicted in more recent iterations as a City Planet, and described as one of the larger Hive Worlds in the Imperium. However, it lacks many of the common fictional features of Mega Cities and City Planets in that it's not really a trading hub, but rather the holiest of holy religious sanctuaries, and the entire planetary surface amounts to a massive, militarised religious compound in which the most trusted servants of the God-Emperor are allowed to live.
Sprawls/metroplexes in Shadowrun.
- Like much of the Shadowrun universe, these are most likely inspired by William Gibson's work, outlined above.
Mort City in SLA Industries.
Though there have been many cities mentioned in Magic: The Gathering, none quite so massive as the plane-spanning city of guilds, Ravnica. A city so big, it has all five mana-producing lands contained within it with room to spare. A city so big, it took three entire card sets to cover it all.

Video Games

Mega-Primus in X-COM: Apocalypse.
One of the playable locations in Mass Effect 3 is a city representing the unification of Vancouver and Seattle.
Kaineng City from Guild Wars Factions.
One of the ultimate goals in any SimCity game is to reach as high a population as possible. In the original game, reaching a population of 100,000 would upgrade your city into a Metropolis. Reaching 500,000 citizens meant that your city be classified as a Megalopolis.

Web Comics

Gunnerkrigg Court: The court itself has gigantic proportions, its full size has yet to be revealed.
Shifters: The comic takes place in Shade City a Mega City that encompasses a vast amount of territory in the Pacific Northwest.
Schlock Mercenary got these, as described here. The city is question is the capitol of Earth built upon an artificial island. Yes, building tips stick out of the clouds a little. Flying Cars are a necessity, and when there's No Escape but Down, it can be 4 km down - not that flight belts aren't only slightly less common.

Western Animation

Avatar: The Last Airbender: The Outer Wall of Ba Sing Se seems to encompass nearly 10% of the land area of the largest continent on its planet, and its population is equally gigantic, especially in comparison to those of the entire Air Nomad and Water Tribe civilizations.
The Jetsons

Real Life

The proper term to describe cities- more specifically, Broad-stroke regions- that have reached extreme levels of population and urban development is Megalopolis. The following examples listed below all qualify, as most of them are a chain of metropolitan areas that are closely connected to each other that they have begun to overlap.
According to The Other Wiki, there are about as many people in Jakarta as there are people in Australia (especially if you count Jakarta's satellite cities and zones).
- There are about as many people in Tokyo as there are people in Canada. Makes a certain trope feels juuuuust a little bit Justified.
- If Tokyo were a seperate country it would be the 34th most populous in the world, after Poland.
In the US (and other places), urban sprawl has started expanding cities to the point where they run together. For instance:
- The first was known originally as BosWash, a contiguous mass of cities and suburbs running from Boston to Washington DC.
- The Los Angeles metropolitan area is currently the largest (in terms of area) metropolitan area in America and the second most populous, with more then 15 million residents.
- South Florida (the metropolitan area based around Miami) is a continuous strip of connected cities running 110 miles, though never more than 20 miles wide. It's the longest unbroken strip of urbanization in America and is home to other 5 million people.
Outside of the western world:
- There are very large cities in China such as Shanghai (20 million) and Beijing (17 million) and there are a number of large cities in the Pearl River Delta such as Shenzhen (12 million) and Guangzhou (16 million) which may one day merge to form the largest mega city in the world [1].
- Mumbai and Delhi/New Delhi in India have populations of over 20 million and over 15 million respectively.
- Cairo (15 million people) and Lagos (12 million) in Africa. Yeah.