Shadow economy
The part of an economy involving goods and services paid for in cash, and therefore not declared for tax. Also called hidden economy, parallel economy, or black economy [Bateman, McAdam, p.19].
A portion of the economy goes undetected, mainly by taxing authorities. Transactions usually are barter or in cash and include both illegal activities and activities that would be legal except for their unrecorded nature [Friedman, p.740].
The shadow economy refers to economic transactions deemed illegal, either because the goods or services traded are unlawful or because transactions fail to comply with governmental reporting requirements.
It is difficult to accurately gauge the size of shadow economies because, by nature, they're not subject to governmental oversight; therefore, the economic activity does not generate tax returns or appear in official statistical reports; however, tracking outgoing expenditures, even though the transactions are cloaked, can give a sense of statistics. In other words, the money spent—not accounted for in recorded transactions—theoretically represents the breadth of black market activity [Investpedia].
Bateman, H., McAdam, K. (2003). Dictionary of Economics. London: A & C Black Publishers Ltd.
Friedman, Jack P. (2012). Dictionary of Business and Economics Terms (Barron's Business Dictionaries) (5 ed.). New York: Barron’s Educational Series.
Underground Economy: Definition, Statistics, Trends, and Examples. Investopedia.Retrieved from: https://www.investopedia.com/terms/u/underground-economy.asp