Public Goods

Goods that are non-rival in consumption and non-excludable, so markets undersupply them without collective action.

Specifications & Metrics

Samuelson's The Pure Theory of Public Expenditure (1954) gave the modern definition.
National defense and lighthouse beams are the standard examples: one user's consumption does not diminish another's.
Free-rider incentives make voluntary payment unsustainable, which is the classic argument for taxation.