Tariff
A tariff is called an optimal tariff if it's set to maximize the welfare of the country imposing the tariff. It is a tariff derived by the intersection between the "trade indifference curve" of that country and the offer curve of another country. In this case, the welfare of the other country grows worse simultaneously, thus the policy is a kind of "beggar thy neighbor policy". If the offer curve of the other country is a line through the origin point, the original country is in the "condition of a small country", so any tariff worsens the welfare of the original country.
Taxation in the United States
The United States imposes tariffs or customs duties on imports of goods. The duty is levied at the time of import and is paid by the importer of record. Customs duties vary by country of origin and product. Goods from many countries are exempt from duty under various trade agreements. Certain types of goods are exempt from duty regardless of source. Customs rules differ from other import restrictions. Failure to properly comply with customs rules can result in seizure of goods and criminal penalties against involved parties. United States Customs and Border Protection ("CBP") enforces customs rules.