A public good is a good which is nonexcludable and nondepletable. The first condition means that it is impossible to exclude consumers from partaking in the good, and the second condition means that one consumer's consumption of the good does not prevent others from consuming it.
Explained another way, a public good is available to all such that consumption by one person does not reduce its availability to others. An example of a public good is national defense, as it protects everyone and its benefits to one person does not diminish its benefits to others.