In economics, an externality or spillover of an economic transaction's an impact on a party that isn't directly involved in the transaction. In such a case, prices don't reflect the full costs or benefits in production or consumption of a product or service. A positive impact's called an external benefit, while a negative impact's called an external cost. Producers and consumers in a market may either not bear all of the costs or not reap all of the benefits of the economic activity. For exampl… (More on Externality)