A slum is a crowded, run-down part of a city where buildings are poorly maintained and people live in difficult conditions. Slums typically lack basic services that most neighborhoods have, like reliable plumbing, electricity, or garbage collection. Streets might be unpaved, buildings might have leaky roofs or crumbling walls, and families often live in very small spaces.

Slums usually develop when cities grow rapidly and can't provide enough affordable housing for everyone who needs it. Throughout history, slums have existed in cities around the world. In 19th-century London, slums were densely packed neighborhoods where factory workers lived in cramped, unsanitary conditions. Today, slums exist in many rapidly growing cities, especially where large numbers of people move from rural areas seeking work.

The word is sometimes used more loosely to describe any neighborhood that looks shabby or neglected, though this casual use can be disrespectful to the people who live there. When someone says they're going “slumming,” they mean visiting a place less comfortable than where they usually spend time, though this phrase can sound insulting.