The term "underworld market" refers to an illegal or black market where goods and services are traded. However, there are several synonymous terms that describe similar, illicit activities. One such phrase is the "shadow economy," which denotes economic activities that are disregarded by government authorities. Another comparable term is the "grey market," which describes the sale of goods through channels that are legal but not authorized by the original manufacturer. Additionally, the "informal economy" refers to economic activities that occur outside of formal business structures. All of these phrases describe various forms of unregulated, off-the-grid marketplaces that exist on the fringe of mainstream economic systems.