Question

what is informal sector

Answer 1

The informal sector, informal economy, or grey economy is the part of an economy that is neither taxed, nor monitored by any form of government. Unlike the formal economy, activities of the informal economy are not included in the gross national product (GNP) and gross domestic product (GDP) of a country.
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Answer 2

Informal sector is the sector of economy which is not controlled by the government.

For example : a private owned shop, an individual owned mill etc.