Megalopolis - conurbation of multiple large cities to form something larger than a metropolis

"A megalopolis, sometimes called a megapolis; also megaregion, city cluster or supercity, is typically defined as a group of two or more roughly adjacent metropolitan areas, which may be somewhat separated or may merge into a continuous urban region. The megalopolis concept became highly influential as it introduced a new, larger scale in thinking about urban patterns and urban growth. - Wikipedia"