Those engaged in underground activities circumvent, escape or are excluded from the institutional system of rules, rights, regulations and enforcement penalties that govern formal agents engaged in production and exchange.Different types of underground activities are distinguished according to the particular institutional rules that they violate.A complementary measure of the unreported economy is the "tax gap", namely the difference between the amount of tax revenues due the fiscal authority and the amount of tax revenue actually collected. A summary measure of the unrecorded economy is the amount of unrecorded income, namely the amount of income that should (under existing rules and conventions) be recorded in national accounting systems (e.g. Unrecorded income is a particular problem in transition countries that switched from a socialist accounting system to UN standard national accounting.
The literature on the black market has not established a common terminology and has instead offered many synonyms including: subterranean; hidden; grey; shadow; informal; clandestine; illegal; unobserved; unreported; unrecorded; second; parallel and black.
The United Nations has reported that the retail market value of illegal drugs is 1.6 billion USD.
Although law enforcement agencies intercept in a fraction of the illegal drugs, and incarcerate hundreds of thousands of wholesale and retail sellers, the very stable demand for such drugs and the high profit margins encourages new distributors to enter the market without a decrease in the retail price.
In truth, the best hope for growth in most emerging economies lies in the shadows.
From the late 19th and early 20th centuries, many countries began to ban the keeping or using of some recreational drugs, such as the United States' war on drugs.