Varian, "Microeconomic Analysis" was the standard grad econ text until
recently. In the 3rd edition p. 414, starting the chapter titled
"public goods", it gives this definition:
"We say that a good is excludable if people can be excluded from
consuming it. We say that a good is nonrival if one person's
consumption does not reduce the amount available to other consumers. ...
Goods that are not excludable and are nonrival are called public goods."
Robin D. Hanson hanson@hss.caltech.edu http://hss.caltech.edu/~hanson/