nonexcludability

In economics, a public good is defined by two key characteristics: non-excludability and non-rivalry. Non-excludability means that it's impossible to prevent individuals from benefiting from the good, even if they don't pay for it. Non-rivalry signifies that one person's consumption of the good doesn't reduce its availability for others.

A classic example is national defense; it protects everyone, and one person's protection doesn't lessen another's. Clean air is also considered a public good for similar reasons. These contrast with private goods, like food, which are both excludable (you can be prevented from having it if you don't pay) and rivalrous (if one person eats it, another cannot).

Because markets tend to undersupply public goods, governments often provide them through taxes and subsidies.1