megacity - Wiktionary, the free dictionary

From Wiktionary, the free dictionary

From mega- +‎ city.

megacity (plural megacities)

1. A very large city; a megalopolis.

   • 2011 July 30, “Cholera and the super-loo”, in The Economist:

     Why, for instance, do water users in a megacity like Lagos not pay an extra charge to get cesspits pumped out?