Quota.

One of the most common ways of establishing a barrier to free trade is to establish a quota for specific goods. A quota establishes a quantitative limit on the amounts of goods that can be imported. Quotas may be expressed in terms of weight (so many tons of grain), value (no more than $1,000,000 in value of a good), units (no more than 1,000 trucks of a certain type), or more recently in terms of a percentage of final value (no more than 50 percent of the final value of a good may come from outside the country). Quotas are often used as retaliatory measures for what a country perceives as unfair trade practices of another nation. [ABSTRACT FROM AUTHOR]