Which term describes a city with more than 20 million inhabitants?

A city with more than 20 million inhabitants is referred to as a megacity. This term is used specifically to describe large urban areas that accommodate a massive population, typically over the 10 million mark, with many exceeding 20 million. Megacities are characterized by significant economic, political, and social influence, often serving as major hubs for global commerce and culture.

The other terms carry different meanings. A metropolis generally denotes a large urban area or the most important city in a region but does not specify a population range. A conurbation refers to an extended urban area, typically formed by the merging of multiple cities or towns, which emphasizes spatial relationships rather than population size. Global cities, while they may overlap with megacities in population, focus more on a city's global influence, connectivity, and role in world affairs rather than just its population size.