An externality is a cost or benefit incurred by a third party who is not directly involved in an economic transaction. This concept highlights the impact that the actions of individuals or firms can have on others, often leading to market failures when these external effects are not accounted for in the decision-making process. Externalities can be positive, such as the benefits of education, or negative, like pollution, and understanding them is crucial for analyzing how markets operate and for devising policies to correct inefficiencies.