The underground economy refers to all productive activities that are legal but remain hidden from public authorities in order to avoid paying taxes (e.g. VAT or income tax) or social security contributions; comply with legal regulations; or comply with official procedures and regulations, such as filling in administrative forms or statistical questionnaires. In addition to "underground economy," which is the most commonly used term, some countries also use the terms "sumerged economy," "hidden economy," or "black economy" to denote this type of activity.