Define 'megacity'.

A megacity is defined as a city that has a metropolitan area population of over 10 million people. This definition highlights the scale and significance of megacities in terms of population, which often leads to considerable impacts on urban development, infrastructure, and social dynamics. Megacities typically experience rapid urbanization and face challenges such as congestion, pollution, and the provision of essential services.

The population threshold of over 10 million is critical as it captures those urban centers that exert substantial influence not only locally but also globally. Such cities play pivotal roles in the economy, culture, and international affairs, significantly shaping trends and developments in various sectors. Therefore, this definition aligns with the characteristics and complexities associated with megacities, setting them apart from smaller urban areas that may not experience the same level of growth and associated challenges.