Customs is the plural of custom, a common practice among a group of people.

Customs duty is a tariff or tax on the import or export of goods. Traditionally in England, it was part of the customary revenue of the king, and therefore did not need parliamentary consent to be levied, unlike excise duty, land tax, or other impositions.

So Customs is an authority or agency in a country responsible for collecting customs duty and for controlling the flow of people, animals and goods, as well as personal effects and hazardous items, in and out of the country. Depending on local legislation and regulations, exporting or importing of some goods may be restricted or forbidden, and the Customs agency is enforcing these rules. The Customs agency also monitors persons leaving and entering the country, checking for appropriate documentation and trying to capture persons identified by international search warrants. As well as increasing the population of a particular country a person may be carrying an infectious disease or may pose a threat to that country.

• Examples: Customs and Excise (in the UK and Hong Kong) or Customs and Border Protection (in the US)
• See also: Border, Border control, Norm (sociology)