Tariff
A tariff is a tax on imports or exports between sovereign states. It is a form of regulation of foreign trade. It is a policy that taxes foreign products to encourage or protect domestic industry. It helps limit trade deficits. The tariff is historically used to protect infant industries and to allow import substitution industrialization.
History of taxation in the United States
While the government was originally funded via tariffs upon imported goods, tariffs now represent only a minor portion of federal revenues. Non-tax fees are generated to recompense agencies for services or to fill specific trust funds such as the fee placed upon airline tickets for airport expansion and air traffic control. Often the receipts intended to be placed in "trust" funds are used for other purposes, with the government posting an IOU ('I owe you') in the form of a federal bond or other accounting instrument, then spending the money on unrelated current expenditures.