A City World is entirely or almost entirely covered with sophont-made structures: cities in other words, millions or often billions of square kilometers of them, often layered kilometers up from the surface. They are rare, but they exist within Charted Space.
There is no standard set of characteristics for an ecumenopolis, however they are more likely on a world with excellent conditions for conventional life. Indeed, in several cases the process of becoming an ecumenopolis changed the Atmosphere and Hydrographics codes of a world's UWP.
Probable Planetary Orbit & Climate
They tend to be in the Habitable Zone.
Most worlds with tens of billions of sophonts, which is as high as most sectors in Charted Space see, can house their population on a small fraction of the world's surface, especially if orbiting habitats are included. Thus, when attempting to boost pride, garner investments, or for some other such reason claim that a world is a ecumenopolis when the case is at best arguable, it is common to include two types of areas other than the traditional residential, commercial, industrial, and city services. 
For truly large numbers of biological sophonts, heat removal and food production take more space than merely housing the population. Thus, these two functions are often counted toward whether a world is mostly covered by "city", even when they do not look like part of a city (such as with vast open agricultural areas, or weather control machinery such as giant radiators and solar shields). (While it is typical for there to be agricultural worlds elsewhere in the system, possibly even moons of the ecumenopolis, these are not counted as part of the "city world" even if the people of said world would starve without constant assistance by the agricultural worlds.)
Long-settled worlds may have large urban areas that have become unusable (contaminated with radiation or biological hazards, a post-ortillery vast jumble of ruin, or considered "unclean" by local religions), and have not yet been cleaned up. In theory this is not a permanent situation, but technically most Ancients Sites qualify as this (even if they rarely cover anywhere close to 1% of a world's surface, let alone a significant portion), demonstrating that such debris can be extremely long-lived in some cases.
Few worlds in Charted Space have enough population to qualify as "city planets" without these additions. Most any world with a Population code of B or above qualifies, but these tend to be away from the major polities. Known examples include Yaskoyloyt in Datsatl Sector, and a few Etra worlds in Hfiywitir Sector. More common examples are Population code A and a small size, such as Irap.
Historical Example: Not A Ecumenopolis
To take one well-studied historical example for which copious data exists (and which is sometimes wrongly assumed by modern students to have been an ecumenopolis), Terra around -2500 - well into Population code 9 and nearing the lower bound of A, but with a Planetary Size of 8, larger than most colonized worlds - had about 3% of its land area covered by cities proper, 37% by farmland, and negligible amounts by weather control or urban ruins. That is not a majority of the land area even counting agricultural areas, and just under 1/3 of Terra's surface is land. (The portions of water area used for aquaculture, industry, and waste dumps do not significantly increase the total world area used.) Although this is a total world area of cities proper of about 1%, all data suggests that merely increasing the population 10 times - as a straight bump of code 9 to A indicates - would lead to significantly less than 10-fold increase in city proper area, agricultural area, or any other area use category. To qualify on cities proper, it would have had to reach Population code B.
Developing planets is costly and often takes decades or even centuries to go from a low population to a dense populations dwelling by hab blocks, population domes, arcologies, and numerously densely populated urban metropolises. Growing substantially beyond this takes many centuries and/or sustained immigration. Further, this level of socioeconomic development requires a variety of sustained big investments in infrastructure and industries.
Some of the core worlds of Charted Space have been settled for literally thousands of years (...perhaps much longer) and have seen internal population growth push them to dense, ecumenopolis level development. Yaskoyloyt has received substantial immigration from coreward for thousand of years, with cultural factors limiting emigration, while the Etra are surrounded by the Aslan Hierate and its client states, limiting the relief valves for their growing population, to the same result.
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