public good

In economics, a public good is something of value, which is hard or even impossible to produce for private profit, because the market fails to account for its large beneficial externalities. By definition, public goods possess two properties:

  • once it has been produced, everyone can benefit from it without diminishing other's enjoyment.
  • once it has been created, it is very difficult to impossible to prevent others from benefiting from the good.

"Pure" public goods possess these properties absolutely. Pure public goods are rare, missile defence would be an example (if it worked).