Final answer:
Excludability refers to the ability to prevent someone from using a good, which contrasts with non-excludable goods that anyone can use, such as public goods like national defense.
Step-by-step explanation:
The property of a good whereby a person can be prevented from using it is known as excludability. When a good is excludable, not everyone has access to it; typically, only those who pay for the good or service may use it. This is in contrast to non-excludable goods, where no one can be excluded and anyone can use the good or service, often without any charge.
Public goods are a common example of non-excludable and non-rival goods, meaning they are freely available to everyone and one person's use does not diminish another's opportunity to use it. For instance, national defense is a public good because it is both non-excludable, as it protects all citizens without the possibility of excluding anyone, and non-rival, since one person benefiting from national defense does not prevent others from also being protected.