Final answer:
A public good is characterized by being nonexcludable and non-rival. It is costly or impossible to exclude someone from using the good, and when one person uses it, it does not prevent others from using it.
Step-by-step explanation:
A public good is characterized by being nonexcludable and non-rival. Nonexcludable means that it is costly or impossible to exclude someone from using the good. For example, national defense is a public good because it is provided to everyone and cannot be excluded to protect only certain individuals. Non-rival means that when one person uses the good, it does not prevent others from using it.
Public goods are difficult for markets to produce because free riders will try to use the public good without paying for it. Measures to ensure payment for public goods include government actions, social pressures, and specific situations where markets can collect payments.