Public Domain has two principal meanings, both of which make explicit references to property and ownership: it can refer to land owned by a unit of government as opposed to private property owned by individuals or corporations. The term can also refer to a realm of information, knowledge or intellectual property that is not protected by copyright, trade mark or patent.
Property in the public domain is sometimes said to be owned by everyone and can be used at any time for any purpose by anyone or everyone. In the language of law, it cannot be alienated. In the terminology of economics, it is said to be non-exclusive. Common examples include the works of William Shakespeare and the art of Leonardo da Vinci. The legal definition of what is in the public domain varies widely across countries, including very different periods of time for the expiry of copyright of materials. This has major implications for internet access to P.D. materials, with some sites determining eligibility to download on the basis of IP addresses and applicable country law.
In legal and constitutional terms, the public domain in this second sense is typically defined by negation: It refers to all those things which cannot be owned, controlled or excluded from others by any individual or entity.