Informal economy is a system of trade or economic exchange used outside state controlled or money based transactions. Practiced by most of the world's population, it includes barter of goods and services, mutual self-help, odd jobs, street trading, and other such direct sale activities. Income generated by the informal economy is usually not recorded for taxation purposes, and is often unavailable for inclusion in gross domestic product (GDP) computations. [Quoted from: BusinessDictionary.com]
Informal economy is an unofficial or underground sector of economic activity beyond government regulation and taxation.