Customs duty is a fee imposed by a government on goods that are imported or exported through a port or airport. This fee helps control the movement of goods across borders and provides governments with a source of revenue. There are several synonyms for the term "customs duty", including import tax, tariff, excise, surcharge, levy, and fee. These terms are often used interchangeably to describe the costs associated with moving goods across international boundaries. Customs duty rates vary by country, and they can be either ad valorem (a percentage of the value of the goods) or specific (a fixed fee per unit). Regardless of the term used, customs duties are an important consideration for anyone involved in international trade.