= Resources that, because of their “public” nature, are difficult or costly to exclude anyone from using. 
1. From the Wikipedia at http://en.wikipedia.org/wiki/Public_good
"In economics, a public good is a good that is non-rival. This means: consumption of the good by one individual does not reduce the amount of the good available for consumption by others. Thus, 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.
Non-rivalness and non-excludability may cause problems for the production of such goods. Specifically, some economists have argued that they may lead to instances of market failure, where uncoordinated markets are unable to provide these goods in desired quantities. These issues are known as public goods problems, and there is a good deal of debate and literature on how significant they are, and on what their solutions might be. These debates can become important to political arguments about the role of markets in the economy. More technically, public goods problems are related to the broader issue of externalities"
2. Peter Suber:
"What is a public good? In the technical sense used by economists, a public good is non-rivalrous and non-excludable. A good is non-rivalrous when it’s undiminished by consumption. We can all consume it without depleting it or becoming “rivals”. Radio broadcasts are non-rivalrous; my reception doesn’t block yours or vice versa. A good is non-excludable when consumption is available to all, and attempts to prevent consumption are generally ineffective. Radio broadcasts are non-excludable for people with the right equipment in the right area. Breathable air is non-excludable for this purpose even though a variety of barriers, from pollution to suffocation, could stop people from consuming it.
Knowledge is non-rivalrous. Your knowledge of a fact or idea does not block mine, and mine does not block yours." (http://www.arl.org/sparc/publications/articles/knowledge-public-good.shtml)