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.