A city is broadly defined as a substantial human settlement, though its specific criteria vary worldwide, typically denoting a densely populated area where members primarily engage in non-agricultural tasks and possess extensive systems for housing, transportation, and utilities. Following two centuries of unprecedented urbanization, over half the global population now lives in cities, profoundly impacting global sustainability and influencing major issues like climate change and global health. Functioning as administrative, commercial, and cultural hubs, cities facilitate interaction and efficiency, forming the core of larger metropolitan areas while also being globally interconnected.

Recognized for their potential to reduce ecological footprints per inhabitant due to density and efficient resource use, cities are central to achieving Sustainable Development Goal 11. However, this concentration also presents significant challenges, including urban heat islands, concentrated pollution, and stressed water supplies, highlighting their complex and dual role in our world.