Wikipedia, I think. In economics, market failure is a situation in which the allocation of goods and services by a free market is not efficient. That is, there exists another conceivable outcome where a market participant may be made better-off without making someone else worse-off. (The outcome is not Pareto optimal.)
Libertarians tend to lose their minds when someone says “market failure,” since in their cosmology, this is an impossibility (Big Brother is double-plus ungood – just doesn’t make sense…).
See Nick Stern’s 2006 framing of climate change as a market failure. We may be beyond that now, I fear….