In economics, what kind of good is non-excludable and non-rivalrous?

In economics, a good that is non-excludable and non-rivalrous is called a public good.

Non-excludability means it is difficult or impossible to prevent people from using the good, while non-rivalry means one person’s use does not substantially reduce its availability to others. National defense is a frequently cited example: protection generally covers residents together, and one person’s protection does not use it up for another.

These characteristics can create a free-rider problem. People may benefit without paying, which can make private firms reluctant to provide the socially desirable amount. Governments therefore often finance or provide some public goods through taxation.

The classification is about economic characteristics, not simply whether government supplies something. A public park may become crowded and therefore partly rivalrous, while a privately supplied service can still have public-good characteristics. Common goods are rivalrous but difficult to exclude people from using, such as an open-access fishery.