Public Good
A good that is non-excludable and non-rival, typically undersupplied by private markets.
What is Public Good?
A public good has two defining characteristics: it is non-excludable (impossible or impractical to prevent non-payers from consuming it) and non-rival (one person's consumption does not reduce the amount available to others). Classic examples include national defense, public streets, free-to-air broadcasts, and lighthouses. Because non-payers cannot be excluded, private markets fail to provide public goods in sufficient quantities — rational individuals free-ride instead of voluntarily paying. This market failure is the primary economic justification for government provision of public goods funded through taxation. In contrast, club goods are excludable but non-rival (e.g., cable TV), while common goods are non-excludable but rival (e.g., ocean fisheries).
Example
The U.S. military provides national defense to all 335 million Americans regardless of whether they pay taxes or are citizens. No one can be excluded from the protection it provides (non-excludable), and one person's protection does not reduce anyone else's (non-rival). No private company could profitably sell national defense, because buyers could free-ride — so the government funds it through compulsory taxation.
Source: Investopedia — Public Good