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 in United States history
Currently only about 30% of all import goods are subject to tariffs in the United States, the rest are on the free list. The "average" tariffs now charged by the United States are at a historic low. The list of negotiated tariffs are listed on the Harmonized Tariff Schedule as put out by the United States International Trade Commission.