Economic Definition of externality. Defined.
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Term externality Definition: A cost or benefit that is not included in the market price of a good because it's not included in the supply price or the demand price. Pollution is an example of an externality cost if producers aren't the ones who suffer from pollution damages. Education is an example of an externality benefit when members of society other than students benefit from a more educated population. Externality is one type of market failure that causes inefficiency.