Public good

In financial side, a public good is a good that is non-rival and non-excludable. This means that utilization of the good by one individual does not reduce the amount of the good available for consumption by others; and no one can be successfully excluded from using that good. For example, if one individual eats a cake, there is no cake left for anyone else, and it is probable to exclude others from consuming the cake; it is a rival and excludable good, or a private good. Conversely, breathing air does not considerably reduce the amount of air available to others, nor can people be effectively excluded from using the air. This makes it a public good, These are highly hypothetical definitions: in the real world there may be no such thing as an completely 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.