Activities that are are productive in an economic sense and completely legal (in terms of compliance with the standards and laws), but that are deliberately concealed from public authorities.

Concealing the activities is in order to avoid: the payment of income tax, VAT or other taxes; the payment of social security contributions; having to meet certain legal standards (such as minimum wags, maximum hours, safety of health standards, etc.); complying with certain administrative procedures (such as completing statitical questionnaires or other administrative forms).