megacity
noun [ C ]
(also mega-city); (mega city) uk/ˈmeɡəˌsɪti/ usplural megacitiesa very large city that has a population of more than 10 million people and that is often made of two or more urban areas that have grown so much that they are connected:
Lagos is a mega-city that now stretches over roughly 300 square kilometres.
London is likely to become one of the world's biggest megacities by 2020.