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.
Tariff
A customs duty or due is the indirect tax levied on the import or export of goods in international trade. In economic sense, a duty is also a kind of consumption tax. A duty levied on goods being imported is referred to as an "import duty". Similarly, a duty levied on exports is called an "export duty". A tariff, which is actually a "list" of commodities along with the leviable rate (amount) of customs duty, is popularly referred to as a customs duty.