When member nations agree to trade completely free between themselves (no tariffs, no embargoes, no quotas) but impose common external trade barriers on all imports coming in from non-member nations, usually a common external tariff
When a country joins a customs union, it can now sell its goods and services without tariffs to all other member nations, creating a huge increase in market size and trade
The benefits of a country joining a customs union include economies of scale, greater market access, technological transfer, and lower prices for consumers