An agglomeration of multiple cities is called a conurbanation.
A conurbation is a region made up of a number of cities, large towns, and other urban areas that, through population growth and physical expansion, have arised in order to form one continuous urban or industrially developed area. Generally, a conurbation is a polycentric urbanised area, in which transportation has developed to link areas to create a single urban labour market or travel to work area
