An externality is a cost of economic activity that is not easily ascertainable, or accounted for, and that therefore is not usually covered by the prices of goods. Externalities are costs that are suffered, but not paid for. Because the market has a hard time recognizing them properly, they are considered a sort of market failure; and the perfection of markets involves accurate accounting for externalities, and for their inclusion among the factors of production, and thus of the prices of goods and services. Perfect markets internalize all costs.
Many externalities are costs imposed on commons: goods held in common by the whole commonwealth – by everybody, and by nobody in particular. Precisely because they are difficult to account for, externalities are not usually controlled costs. The commons they injure are then subject to unbridled exploitation.
Imports have obvious benefits, or no one would want to pay for them. They also impose externalities.