What does externality mean?
An externality is a consequence of an economic activity that falls on people who had no say in the decision behind it and are not compensated for it. When a factory pollutes a river, downstream residents bear health and clean-up costs that never appear on the factory's balance sheet — this is a negative externality. Conversely, when someone plants a garden that beautifies a whole street, neighbours benefit without paying: a positive externality. Because such costs and rewards escape market prices, economists treat externalities as a classic cause of market failure, arguing that governments should intervene — through taxes, subsidies, or regulation — to 'internalize' them so prices reflect true social costs. Beyond economics, the word has an older, more abstract sense in philosophy, referring to existence outside the self or mind. Formal and largely academic in tone, externality is indispensable vocabulary for anyone discussing policy, environment, or public goods.
nounA consequence of an economic activity that affects third parties who did not choose to incur it, and which is not reflected in market prices. More generally, a quality or feature belonging to or arising from outside a thing.
- (Economics) A cost or benefit imposed on third parties by an economic transaction without compensation, e.g. pollution as a negative externality or vaccination benefits as a positive externality.
- (General/philosophical) The state or fact of existing outside oneself; external existence or objectivity.
"Pollution from the factory is a classic negative externality, since nearby residents bear costs they never agreed to."
"Carbon emissions are a negative externality: drivers enjoy cheap fuel while society pays for climate damage."
"Philosophers debate whether the mind can truly know anything about externality beyond its own perceptions."
The plural is standard and frequently used when discussing multiple third-party effects at once.
"Economists catalogue the many externalities generated by industrial production."
Economists call pollution a 'market failure' because its price tag lands on strangers — the externality is the cost nobody signed up for.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of externality
Externality derives from the Latin adjective 'externus', meaning outward or external, formed from 'exterus' (on the outside), itself rooted in the comparative of 'ex', meaning out of. It entered English in the 17th century via the adjectival form 'external', which came through Latin into Middle French before being adopted into English. The philosophical sense of external existence developed first, with the modern economic sense — a third-party cost or benefit — emerging much later, becoming central to economic thought in the 20th century through the work of economists such as Arthur Pigou. Cognates sharing the same Latin root include 'exterior' and 'extrinsic'.
How externality is actually used
Predominantly formal and academic register — most common in economics, environmental policy, and philosophy. In everyday conversation, speakers usually prefer 'side effect' or 'spillover'. Often qualified as positive or negative depending on whether the third-party impact helps or harms.
Easily confused with externality
Externalization is the act of pushing costs or processes outside oneself or an organization, while an externality is the resulting side effect borne by uninvolved third parties.