Externalities occur when the actions of individuals or firms have unintended consequences on third parties who are not part of the original transaction. These external effects can be positive (benefits) or negative (harms), and they are not typically accounted for in the market transaction.

For example:
- A factory that pollutes a river may harm the local ecosystem and the people who depend on it, causing negative externalities.
- On the other hand, a homeowner who maintains a beautifully landscaped garden might enhance the aesthetics of the neighborhood, creating positive externalities for the neighbors.

   The reason externalities are a significant economic concept is that they can lead to market failures. When externalities exist, the market equilibrium does not reflect the full social costs or benefits of an activity, leading to inefficiencies. 

To address externalities, governments may intervene through regulations, taxes, subsidies, or other policy measures to internalize these external costs or benefits and achieve a more socially optimal outcome.