A grey market or dark market is the trade of a commodity through distribution channels that are not authorized by the original manufacturer or trade mark proprietor. Grey market products are products traded outside the authorized manufacturer's channel.

A black market, underground economy, or shadow economy is a clandestine market or series of transactions that has some aspect of illegality or is not compliant with an institutional set of rules. If the rule defines the set of goods and services whose production and distribution is prohibited or restricted by law, non-compliance with the rule constitutes a black-market trade since the transaction itself is illegal. Such transactions include the illegal drug trade, prostitution, illegal currency transactions, and human trafficking.