Excludability

Paying customers are given a ticket that would entitle them to a single showing of the movie, and this is checked and ensured by ushers, security and other employees of the cinema.

A good, service or resource that is unable to prevent or exclude non-paying consumers from experiencing or using it can be considered non-excludable.

Public goods will generally be underproduced and undersupplied in the absence of government subsidies, relative to a socially optimal level.

This is because potential producers will not be able to realize a profit (since the good can be obtained for free) sufficient to justify the costs of production.

Brito and Oakland (1980) study the private, profit-maximizing provision of excludable public goods in a formal economic model.

Also taking distribution costs and congestion effects into account, Schmitz (1997) studies a related problem, but he allows for general mechanisms.

Air, whether it is clean or polluted, cannot exclude anyone from its use, and so it is considered a non-excludable "good". A good can be non-excludable regardless of how desirable it could be to be excluded from consuming it (such as smog or pollution in a city).
Public transit (bus) farebox, Vancouver