The tragedy of the commons is a phrase derived from an article of the same name by Garret Hardin, an ecologist, published in 1968.
According to Hardin, if a resource that is considered to belong to a large group of people and there are no constraints on how much of it can be used by each of them is destined to be over-exploited and destroyed. Each person uses as much of the resource as is required to fulfill her/his requirements and other are expected to restrain their consumption to ensure that the resource is not completely destroyed. Though rationally each of those using the resource should ensure that the resource is used in a manner that allows it to replenish itself for their benefit this is not what happens.
Examples of this situation include the rapid decrease in the number of fish when there is no restriction placed on how much each individual is allowed to catch. In the long-term there are no fish left for any of them to catch.