compare Externality, positive etc.
Wikipedia: In economics, a public good is a good that is both non-excludable and non-rivalrous in that individuals cannot be effectively excluded from use and where use by one individual does not reduce availability to others.[1] Gravelle and Rees: “The defining characteristic of a public good is that consumption of it by one individual does not actually or potentially reduce the amount available to be consumed by another individual”. It also can be described as something that is useful for the entire population as is education and infrastructure. This is in contrast to acommon good which is non-excludable but is rivalrous to a certain degree.
marchudson.net 