Final answer:
A good is called (1) rival when its use by one person prevents another person from using it simultaneously. An example of a rival good is a slice of pizza. Public goods are non-rival and non-excludable, like national defense.
Step-by-step explanation:
When a good is rival, it means that if one person uses that good, another person cannot use it at the same time. This is because the use of the good by one person diminishes the availability of the good for another person. A typical example of a rival good is a slice of pizza; if one person eats a slice of pizza, others cannot consume that same slice.
On the other hand, a public good is characterized by being non-rival and non-excludable, meaning one person's use does not prevent usage by others, and it is not easy to exclude someone from using it. National defense is an example of a public good, as one person benefiting from national defense doesn't reduce the benefit to someone else.
Goods can also be excludable, which refers to the ability of providers to prevent those who do not pay from using the good. Cell phone service is an example of an excludable good because providers can deny service to those who do not pay.