public good

public good: Definition and Much More from Answers.com:

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Paul A. Samuelson is usually credited as the first economist to develop the theory of public goods. In his classic 1954 paper The Pure Theory of Public Expenditure[2] he defined a public good, or as he called it in the paper a 'collective consumption good', as follows:

...[goods] which all enjoy in common in the sense that each individual's consumption of such a good leads to no subtractions from any other individual's consumption of that good...

This is the property that has become known as Non-rivalness. In addition a pure public good exhibits a second property called Non-excludability: that is, it is impossible to exclude any individuals from consuming the good.

The opposite of a public good is a private good, which does not possess these properties. A loaf of bread, for example, is a private good: its owner can exclude others from using it, and once it has been consumed, it cannot be used again.

A good which is rival but non-excludable is sometimes called a common pool resource(공유재). Such goods raise similar issues to public goods: the mirror to the public goods problem for this case is sometimes called the tragedy of the commons. For example, it is so difficult to police deep sea fishing that the world's fish stocks can be seen as a non-excludable resource, but one which is finite and diminishing.

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As well as public goods there can be public bads that have negative externality effects instead of positive ones. For example, pollution or political corruption may be bads that show some of the same non-excludability and non-rivalness properties."