Answer
non-rival
non-excludable
Work Step by Step
A good can be called public good if it possesses the characteristic of $\textbf{non-rivalry}$ and $\textbf{non-excludability}$.
This means one person's use of the good doesn't reduce its availability to others (nonrival), and it's difficult or impossible to prevent people from accessing it (nonexcludable). Classic examples include clean air, national defense, and public parks.
