Externalities (costs excluded from transaction costs)
Externalities are costs not captured by the market – e.g., pollution from the burning of fossil fuels that results in contaminated fish and air that causes respiratory illnesses and lost revenues (e.g., from fishing). These costs are not reflected in market prices at say the gas pump or the electricity meter, but rather arise in another system such as healthcare. Negative externalities are considered by economists as "market failures" that distort the market and the economic landscape.
Externalities may be accounted for through the use of pricing mechanisms, such as a tax, that raises the price, which reduces demand and, therefore, the quantity of the goods produced. This is the environmental and economic basis for the idea of the proposed carbon tax or fossil fuel divestment, that seeks to pull investment funds out of fossil fuel companies to force those companies to change some aspect of their business model, or to cause them to fail altogether.
(CC Image (left) by Struthious Bandersnatch (Own work) [CC0], via Wikimedia Commons)
A simple graphical illustration of of the economics of externalities is available here at the Khan Academy.