Public good
Etymology
The economic sense sharpened in mid-20th-century economics, notably Paul Samuelson’s 1954 formalization of goods that are “non-rival” (my use doesn’t reduce yours) and “non-excludable” (you can’t fence people out). “Public” traces to Latin “publicus” (“of the people”). The technical meaning differs from the loose phrase “the public good” meaning general welfare.
noun phrase · /PUB-lik good/
A good that people can use without depleting it for others and that no one can easily be excluded from enjoying.
What it names
The class of things — clean air, national defense, street lighting — that markets underprovide because no one can be charged for them.
Example
A lighthouse is a textbook public good: one ship’s use of the beam doesn’t dim it for another, and none can be billed.
Related idea
A public good’s non-excludability creates the free-rider problem, since people can enjoy it without paying; contrast a positional good, whose value depends precisely on others being excluded.