A tariff is either (1) a tax on imports or exports (trade tariff) in and out of a country, or (2) a list or schedule of prices for such things as rail service, bus routes, and electrical usage (electrical tariff, etc.).

The word comes from the Italian word tariffa "list of prices, book of rates," which is derived from the Arabic ta'rif "to notify or announce."

Tariffs, often called customs, were by far the largest source of United States federal revenue from the 1790s to the eve of World War I, until they were surpassed by income taxes.

Neoclassical economic theorists tend to view tariffs as distortions to the free market. Typical analyses find that tariffs tend to benefit domestic producers and government at the expense of consumers, and that the net welfare effects of a tariff on the importing country are negative. Normative judgments often follow from these findings, namely that it may be disadvantageous for a country to artificially shield an industry from world markets, and that it might be better to allow a collapse to take place. Opposition to all tariffs is part of the free trade principle; the World Trade Organization aims to reduce tariffs and to avoid countries discriminating between differing countries when applying tariffs.