A hive city or simply a hive, is a massive arcology, or fully self-contained city, that is home to millions or even billions of Human beings loyal to the Imperium of Man. Hive cities are often found in clusters on the densely populated urban planets known in the official Imperial lexicon as Hive Worlds.
Given the massive population of most Hive Worlds, this means that their surfaces are usually reduced to industrial wastelands covered in toxic deserts where nothing can live or grow save for the hardy mutants who call these "ash wastes" home and often raid the covered train-like transports that move passengers and cargo between hive city clusters.