A city is a type of settlement that sapient species create. They were often large and complex. Cities can be built on planets or on Space stations. Exact elements of cities depend on the species that create them.
The word "urban" is often used to describe city areas.
For a list of human cities, see here here
Species of the Covenant
Forerunners built their own cities. There were quite a few different types of cities.
Forerunners also built cities on planets, some of which survived the nearly 100,000 year gap from the firing of the Halo Array like the Ten Cities of Edification. Other ground cities of the Forerunners were the ones built on Triniel.
During the development of Halo 3, a special visual look-development level called "The Lost City" was created. This featured a Forerunner city composed of a single building. It was eventually cut but the exterior design was reused in the Halo Legends short Origins.
The map Icebox of the cancelled Halo Online featured the real world city of Tyumen, Russia. A cut map for the game was also supposed to be set in Seongnam, Korea but wasn't implemented before the game's cancellation .