Defining Public Good

From Wikipedia:

In economics, a public good is a good that is non-rivalrous. This means: consumption of the good by one individual does not reduce the amount of the good available for consumption by others.[1] For example, if one individual eats a cake, there is no cake left for anyone else; but breathing air or drinking water from a stream does not significantly reduce the amount of air or water available to others.

The term public good is often used to refer to goods that are non-excludable as well as non-rival. This means it is not possible to exclude individuals from the good's consumption. Fresh air may be considered a public good as it is not generally possible to prevent people from breathing it. However, technically speaking such goods should be called pure public goods. These are highly theoretical definitions: in the real world there may be no such thing as an absolutely non-rival or non-excludable good; but economists think that some goods in the real world approximate closely enough for these concepts to be meaningful.