An informal economy is economic activity that is neither taxed nor monitored by a government, contrasted with a formal economy. The informal economy is thus not included in that government’s Gross National Product (GNP). Although the informal economy is often associated with developing countries, all economic systems contain an informal economy in some proportion.

Informal economic activity is a dynamic process which includes many aspects of economic and social theory including exchange, regulation, and enforcement. By its nature, it is necessarily difficult to observe, study, define, and measure. No single source readily or authoritatively defines informal economy as a unit of study.

The terms “under the table” and “off the books” typically refer to this type of economy. The term black market refers to a specific subset of the informal economy. The term “informal sector” was used in many earlier studies, and has been mostly replaced in more recent studies which use the newer term.

Financial economics is the branch of economics studying the interrelation of financial variables, such as prices, interest rates and shares, as opposed to goods and services. Financial economics concentrates on influences of real economic variables on financial ones, in contrast to pure finance. It centres on managing risk in the context of the financial markets, and the resultant economic and financial models. It essentially explores how rational investors would apply risk and return to the problem of an investment policy. Here, the twin assumptions of rationality and market efficiency lead to modern portfolio theory (the CAPM), and to the Black–Scholes theory for option valuation; it further studies phenomena and models where these assumptions do not hold, or are extended. “Financial economics”, at least formally, also considers investment under “certainty” (Fisher separation theorem, “theory of investment value”, Modigliani-Miller theorem) and hence also contributes to corporate finance theory. Financial econometrics is the branch of financial economics that uses econometric techniques to parameterize the relationships suggested.