Mega city

The term Mega City currently refers to a single city that has a population of over 10,000,000,000. While most mega ciries are planet based, at least 6 of the 50 largest ones within the United Stars of the Galaxies are based on a moon in orbit of a garden planet. There are also some examples that started out as bases on an asteroid, as well as a couple that started out as space stations.

A planet based city is also called a Mega city if it occupies an area equal or greater than 10,000,000 km2.

It's not unusual for Mega cities to contain multiple arcologies.