Русско-англо-русский юридический словарь

A black market, underground economy, or shadow economy is a market characterized by some form of noncompliant behavior with an institutional set of rules. (Wikipedia)

The shadow economy is perhaps best described by the activities of those operating in it: work done for cash, where taxes aren’t paid, and regulations aren’t strictly followed. (The Wall Street Journal)

The shadow economy refers to all work activity and business transaction that occur ‘below the radar’ – economic activity that is undeclared and for which taxes that should be paid are not. (Market Business News)