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.
Customs war
A Customs war, also known as a "toll war" or "tariff war", is a type of economic conflict between two or more states. In order to pressure one of the states, the other raises taxes or tariffs for some of the products of that state. As a reprisal, the latter state may also increase the tariffs.