Key definitions

The key definitions used to discuss urban areas, urbanisation and urban growth are subject to much debate. For the purposes of this topic guide, selected concepts are defined below.

Citizen-led co-production refers to the joint development of public services by citizens and state. Many urban poor organisations have sought to shift engagement with government from making demands to offering partnership, in recognition that state agencies cannot fulfil their obligations alone, and organisations and federations of the urban poor can facilitate, design and implement cheaper and more effective responses (IIED, 2008).

Peri-urban areas are those that immediately border an urban area, between the suburbs and the countryside. In industrial or post-industrial countries the peri-urban is a zone of social and economic change and spatial restructuring, while in much of the global south, the peri-urban is often a zone of chaotic urbanisation leading to sprawl. Peri-urban zones can be seen as new multi-functional territories. Common features include relatively low population density, scattered settlements, high dependence on transport for commuting, fragmented communities, and lack of spatial governance (Ravetz et al., 2013).

Secondary city is a term used to describe the second-tier level of a hierarchical system of cities based on population thresholds. Its meaning varies, and can relate to: population size, administrative area, or political, economic and historical significance of a system of cities below the primary order of cities within a country or geographic region. Secondary cities perform important functions in the national and global system of cities. They are secondary hubs in a complex network of production-distribution supply chains and waste-management recovery systems, connecting different spatial levels of human settlement (Roberts, 2014).

System of cities acknowledges that it is the relationship among cities, their comparative and complementary expertise, and their evolution in relation to other urban and rural areas that should be the focus of national policy (Clarke Alvarez et al., 2008).

Urbanisation is the gradual shift of relative populations from rural to urban areas. Levels of urbanisation are measured crudely by the percentage of population residing in urban areas, and the rate of urbanisation as the percentage increase in urban population (UNDESA, 2014). The sources of urban population growth include: rural to urban migration; natural increases in the population already residing in urban areas, and the urbanisation of rural and peri-urban settlements. From a demographic perspective, urbanisation has a people-centred focus (McGranahan & Satterthwaite, 2014).

Urban areas are those that have a high population density and built-up features compared to the surrounding areas. The term can apply to industrial zones and related infrastructure as well as to cities and towns.

Urban development is the social, cultural, economic and physical development of cities, and the underlying causes of these processes.

Urban growth is an increase in the absolute size of an urban population. This could be at the level of an individual settlement or a collection of settlements (e.g. at the national level). Urban growth and urbanisation often occur together, but not always. A nation’s urban population can grow in absolute terms without increasing in relative terms (Fox & Goodfellow, 2016: 6).

Urban expansion refers to an increase in the built-up area of a settlement or collection of settlements (e.g. at the national level). This often accompanies an increase in urban population size (i.e. urban growth). But urban growth can happen without expansion in contexts of increasing habitation density; conversely urban expansion can occur without urban growth where de-densification happens – e.g. suburbanisation (ibid.).