In a (n), cities and suburbs are so close together that they form one big urban area.

The answer is Megalopolis

Its typically defined as a chain of roughly adjacent metropolitan Area. The term was used by Patrick Geddes in 1915 in his books Cities of Evolution.

One of the example of a Megalopolis is the BosWash Megalopolis, a chain of metropolitan Area that start at Boston through new york, Philadelphia, baltimore, and end in Washington DC