The phrase ‘tragedy of commons’ is in the context of

Concept: A common-pool resource is one that is non-excludable but rivalrous: no user can practically be shut out of it, yet every unit one user takes is a unit no one else can take. Where such a resource is left in open access with no rule limiting use, each user captures the whole private gain from one extra unit of use while the cost of the resulting depletion is spread across all users, so the balance of private incentives tilts towards more use. Individually rational decisions then add up to collective over-use, and the shared stock is run down.

Application: The phrase names precisely that outcome for stocks that renew themselves — open pasture, an ocean fishery, a groundwater aquifer, a village woodlot. Being renewable, each such stock can support harvest indefinitely up to its regeneration rate; being open to all, it carries no automatic mechanism holding total withdrawal at that rate, unless its users are bound by regulation, custom or a collective governance arrangement of the kind Elinor Ostrom later documented. Once withdrawal exceeds regeneration, the stock shrinks year on year even though nobody intended to destroy it. So the phrase sits in the context of the degradation of renewable, free-access resources.

Contrast: The other categories offered are defined by different mechanisms:

• A poisonous-gas release is a single accidental emission from one identifiable operator; there is no shared regenerating stock and no set of competing users drawing it down.

• Tragic conditions of poor people describe the welfare of a group of persons; the word "commons" in the phrase refers to shared resources, not to a class of people.

• Climate change is frequently discussed as an extension of the same incentive logic to the atmosphere treated as a global sink, but that is an application of the idea to one phenomenon, not the context in which the phrase itself is defined.

Result: The phrase belongs to the degradation of renewable free-access resources — the sense in which Garrett Hardin introduced it in Science in 1968.