Definition: When a country limits or entirely prevents trade with another country.
Why do countries use them?
To convince another country to take a certain political action, such as stopping human rights abuses or the development of nuclear weapons.
How are they used?
In 1979, the United States banned imports from Iran after a group of Iranian college students took fifty-two American diplomats and citizens hostage in the U.S. embassy in Tehran—the so-called Iran Hostage Crisis. Partly as a result of the ensuing increase in hostilities, in 1992 the U.S. Congress passed the Iran-Iraq Arms Nonproliferation Act, which outlawed transferring goods or technology to Iran if those products could be used to build a nuclear weapon. The European Union supported the United States on some of those sanctions. The effects of the sanctions varied: while Iran has built an economy in resistance to these sanctions, it has experienced serious setbacks. Because of the difficulty in transferring money into and out of the country, for example, Iran experienced shortages of non-sanctioned products such as cancer medications in 2012.
For the most part, countries put up trade barriers to make it easier to sell their goods abroad or at home, and a variety of economic and political developments can make countries prioritize security, politics, or domestic industry over free trade. But these trade barriers almost always come with unintended consequences and do not always accomplish their goals.
In the end, trade is always going to be messier than the open road the WTO is supposed to protect, but as globalization continues to bring markets closer together, every country is going to have to grapple with the implications of free trade and the barriers that come with it.