A term from economics. The opposite of a private good, like property. A public good is said to be NonExcludable, and NonRival.
NonExcludable goods are things (like the air) which it are difficult to fence people out of. Open Source projects strive to keep their outputs (the source code) as open as possible. The defining feature of proprietary source code are the fences around the code that limit access.
NonRival goods are things (like a fireworks display) which tend not to suffer any decline in quality from the arrival of each additional consumer of the good. Open source is totally nonrival. Should one person install and use the software it never reduces the quality of function being experianced by another who has installed the software.
The lack of a fence around the public good makes it very hard to charge for it. For this reason non-profit entities, like the ApacheSoftwareFoundation, often take responsiblity for the costs of creating public goods.
The nonrival nature of public goods tends to eliminate the need to compete for access to the good. That can make it much easier to cooperate in their provisioning.
The economic's liturature is full of stories of sad things around the edges of public goods.