Externality
An externality is a cost or benefit to a third party as a result of someone else’s actions. Externalities lie outside the market system. Polluted air, caused by a chemical plant’s emissions, is a negative externality. A common textbook example of positive externalities involves beehives next door to an orchard: the nectar feeds the bees, which in turn pollinate the trees. See also Coase theorem, free rider, Pigouvian taxes and public goods.