An Ecumenopolis is a city which occupies the entire surface area of the planet or moon it is on. The ecumenopolis is the staple of type III civilizations, with every known type-III civilization having at least one.
These cities are typically built in layers, with the bottom layer on the surface of the planet. Each layer is usually over a kilometer apart from the layer above and below it. These cities typically have well over 100 layers each, with some of the largest ecumenopolises having over 150 layers.
These cities have trillions of construction drones milling along the vast support network, filling in any cracks or fissures as they go.
Any oceans that exist on the surface of the planet are either polluted to the point of being useless or completely gone, used for drinking water by the vast populations. If the oceans are still there, they are buried under many layers, with a complex system of catwalks above them.
An ecumenopolis is usually devoid of any meaningful amount of plant life, so oxygen has to be synthesized artificially. Every ecumenopolis either creates oxygen via electrolysis of water or using cyanobacteria. A city like this goes through more oxygen a year than there was oxygen on earth in the 21st century. Which means that the planet needs to produce a ludicrous amount of oxygen.
The city requires exorbitant amounts of energy. This energy is used for a number of things, such as transportation, feeding the population, and waste disposal. The waste heat of an Ecumenopolis from these activities is very great, so great that it would make the planet uninhabitable of the problem weren't addressed. An Ecumenopolis usually has a radiation network on its surface. This network is a high-surface-area metal which is able to radiate the waste heat into space. It is best described as a massive heatsink for the entire planet.
By definition, an ecumenopolis has over three trillion residents. The largest of these cities have well over a quadrillion residents.
Life on an Ecumenopolis
The lower layers of every ecumenopolis are always worse, with some being inhospitable. The bottom layers of most Ecumenopolises are polluted to the point of being absolutely toxic to most forms of life.
The upper layers are much better, with property on the highest levels costing much more than the property on the lower to middle levels.