A slum is a poor area of a big city. A slum is usually overcrowded and dirty, a discouraging place to live.
Some of a city's neighborhoods are fancy and full of wealthy people, while others are slums (and most are somewhere in between). In a slum, most people are poor. The living conditions are inferior in a slum, which may not have the stores, restaurants, and health services its residents want or need. In the early 1800's, the term was back slum, which meant "back alley" or "street of poor people," and was eventually shortened to simply slum.