A trade barrier is an intervention that restricts trade, including by imposing costs.
"Protectionism" means trade limitation, the doctrine of having trade barriers.
Three basic types of trade barriers: tariffs, quotas, and regulation having costs that exceed benefits (including relative to other policy).
A tariff is a trade barrier consisting of a sales tax on imported goods.
Free trade is the exchange of goods without any trade barriers.
Complete free trade has no trade barriers on domestic as well as foreign trade.
Any policy that raises marginal costs without direct compensating benefits is a trade barrier.