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In economic terms, a public good is a produced good or service that is widely available to consumers. In defining a public good, the item will usually be referred to as non-rivalous, non-excludable, or both. The identification of an item as a public good is normally for purposes of analysis, since it is very hard to find goods created for sale to consumers that do not match this criteria.
When a public good is said to be non-rivalous, that simply means that the item remains widely available for consumption by all consumers, even when one consumer had engaged in consumption of the good. Non-rivalous goods can be thought of as easily renewed, or so plentiful that the consumption by one consumer in no way inhibits consumption by others. An example would be an ear of corn picked from a cornfield. While the one ear has been consumed, there are still many other ears of corn that are available for consumption.
A public good is also often classed as being non-excludable. This means that just about anyone can make use of the good in some manner, essentially making that public good universal. Public services are a good example of non-excludable goods, since anyone can receive benefit from the presence of a police force or a fire department, regardless of their condition or economic status.
There are a few basic examples of products that do not meet the basic definition of a public good. One has to do with obtaining professional services, such as those of a doctor or lawyer. When an individual makes an appointment with either of these professionals, he or she is effectively buying the time of that professional. That same time cannot be consumed by any other individual, thus making the duration of the appointment excludable and rivaled. In like manner, many drugs are limited when it comes to consumer access, with some requiring a prescription by a qualified medical professional. The fact that some are excluded from access to those drugs means that medications of this type are considered excludable and rivaled, and thus not a public good.
Over time, advances in technology have created new types of public goods. The electric powered street light is an example of a public good that became common in the early years of the twentieth century. Since its light was available for anyone to enjoy as they walked down a street, the device met the criteria of being non-excludable and non-rivaled. Today, products such as software packages are often classified as public goods. This is particularly true with products such as free software that is widely available to anyone who wishes to use it, with no barriers of cost or economy to inhibit the consumption.