A public good has the opposite characteristics.
National defence is the commonly cited example of a public good.
If each individual acted in complete isolation in buying goods and services, for many public goods there would result an oversupply, e.g. a surfeit of lighthouses, street lights, police protection. For others no supply because no individual owns sufficient resources e.g. nuclear umbrella. Society has long recognised the need for collective action to ensure public goods are produced. Governments should apply the marginal efficiency criteria in deciding which public goods and what quantities of each should be produced. When externalities exist market prices will not lead to economic efficiency because of the divergence between private and social costs.