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.
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.