Result of the concentration on a limited space of constructions, activities or population.

« Population agglomérée » (agglomerated population) (France) : fraction of the population of a municipality living in groups of dwellings separated from each other by less than 200 meters and comprising at least 50 inhabitants (as opposite to « population dispersée » (scattered population) which lives in hamlets (« hameaux » or « écarts »).
Urban agglomeration : cluster of continuously built constructions (in France, separated by less than 200 meters) and hosting a number of inhabitants larger than the statistical threshold defining an urban settlement (in France, 2 000 inhabitants). When the continuously built territory overlaps administrative borders, the agglomeration is called “multi-“ or “pluri-communale” (multi- or pluri-municipal). The agglomeration is a more relevant geographical framework than the municipality when studying a city. In some countries (Denmark), built space alone defines the morphological agglomeration, while in France, the urban agglomeration is adjusted on municipal limits and includes all municipalities whose majority of population lives in the agglomeration.
Agglomeration economies : external economies (see externalities) made by firms thanks to collective use of transport and communication infrastructure and of urban services. Reduction of costs, added to advantages obtained by proximity of a large market, explain that industrial and tertiary firms concentrate in the largest cities, generating a cumulative development of those cities. The agglomeration also facilitates circulation of capital and diversification of job market. By multiplying contact probabilities, it increases the speed of adoption of innovations. Agglomeration advantages, of various nature, are sometimes compensated by increased costs, and problems of congestion and pollution (agglomeration diseconomies).