贫民窟又称为贫民区（slum），一般指穷人居住之所，联合国人类居住规划署将其定义为「以低标准和贫穷为基本特征之高密度人口聚居区」。

这一词语传统上指曾一度有名望，但随着原住民搬迁到城市中更新更好地区而衰落的屋区；现在此词也包括发展中地区城市中有大量临时居所的部份。

贫民窟效应为同群效应（peer effect）的一种社会现象表达，也是复杂系统幂次法则（Power Law）的一种，一般发生在老旧或相对邻近低价社区，由住民因经济力自然搬迁交互作用的结果，由于经济力强者持续迁出、经济弱势持续迁入，并依各自经济能力塑造居住品质，导致演替加剧。

A slum is a heavily populated urban informal settlement characterized by substandard housing and squalor. While slums differ in size and other characteristics from country to country, most lack reliable sanitation services, supply of clean water, reliable electricity, timely law enforcement and other basic services. Slum residences vary from shanty houses to professionally built dwellings that because of poor-quality construction or provision of services have deteriorated into slums.