Urban decay (also known as urban rot, urban death or urban blight) is the sociological process by which a previously functioning city, or part of a city, falls into disrepair and decrepitude. There is no single process that leads to urban decay which is why it can be hard to encapsulate its magnitude.
Urban decay can include the following aspects:

Industrialization
Deindustrialization
Depopulation or overpopulation
Counterurbanization
Economic Restructuring
Abandoned buildings or infrastructure
High local unemployment
Increased poverty
Fragmented families
Low overall living standards or quality of life
Political disenfranchisement
Crime (e.g., gang activity, corruption, and drug-related crime)
Large and/or less regulated populations of urban wildlife (e.g., abandoned pets, feral animals, and semi-feral animals)
Elevated levels of pollution (e.g., air pollution, noise pollution, water pollution, and light pollution)
Desolate cityscape known as greyfield land or urban prairieSince the 1970s and 1980s, urban decay has been a phenomenon associated with some Western cities, especially in North America and parts of Europe. Cities have experienced population flights to the suburbs and exurb commuter towns; often in the form of white flight. Another characteristic of urban decay is blight - the visual, psychological, and physical effects of living among empty lots, buildings, and condemned houses.
Urban decay is often the result from a combination of inter-related socio-economic conditions, including urban planning decisions, economic deprivation of the local populace, the construction of freeways and railroad lines that bypass or run through the area, depopulation by suburbanization of peripheral lands, real estate neighborhood redlining, and immigration restrictions.