The concept of town can be understood with reference to village. Population size is not the only criteria. Specific functions such as manufacturing, retail and wholesale trade, and professional services exist in towns. A city may be regarded as a leading town which has outstripped its local or regional rivals. The city is in fact the physical form of the highest and most complex type of associative life. Cities are much larger than towns and have a greater number of economic functions. The term conurbation is applied to a large area of urban development that emerges due to the merging of originally separate towns or cities. The number of million cities in the world were 162 in the mid 70's and by 2030, a projected 662 cities will have at least 1 million residents. Megalopolis is a Greek word that means "great city. It signifies 'super metropolitan region extending as union of conurbations.