+An **externality** occurs when an economic activity imposes a [Cost](/wiki/cost) or confers a [Benefit](/wiki/benefit) on an unrelated third party. This effect, unreflected in the market price, often leads to [Market Inefficiency](/wiki/market_inefficiency).
+## See also
+- [Market Failure](/wiki/market_failure)
+- [Public Good](/wiki/public_good)
+- [Coase Theorem](/wiki/coase_theorem)
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