The two main characteristics of a public good are

Public goods are another area the government needs to intervene in the free market and they are characterized as nonrivalrous and nonexcludable goods. Nonrivalrous goods are ones that do not reduce in availability as they are used, like a public park which is open to the public and the use by one party of the park does not make it unavailable for others to enjoy. Nonexcludable goods are those that cannot prevent others from benefiting from the advantages they provide, such as the military's defense of the nation which protects everyone and not just specific individuals.