The public domain is a range of abstract materials—commonly referred to as intellectual property—which are not owned or controlled by anyone. The term indicates that these materials are therefore "public property", and available for anyone to use for any purpose. The public domain can be defined in contrast to several forms of intellectual property; the public domain in contrast to copyrighted works is different from the public domain in contrast to trademarks or patented works. Furthermore, the laws of various countries define the scope of the public domain differently, making it necessary to specify which jurisdiction's public domain is being discussed.

The public domain is most often discussed in contrast to works whose use is restricted by copyright. Under modern law, most original works of art, literature, music, etc. are covered by copyright from the time of their creation for a limited period of time (which varies by country). When the copyright expires, the work enters the public domain. It is estimated that currently, of all the books found in the world's libraries, only about 15% are in the public domain, even though only 10% of all books are still in print; the remaining 75% are books which remain unavailable because they are still under copyright protection.

The public domain can also be defined in contrast to trademarks. Names, logos, and other identifying marks used in commerce can be restricted as proprietary trademarks for a single business to use. Trademarks can be maintained indefinitely, but they can also lapse through disuse, negligence, or widespread misuse, and enter the public domain. It is possible, however, for a lapsed trademark to become proprietary again, leaving the public domain.

The public domain also contrasts with patents. New inventions can be registered and granted patents restricting others from using the inventions without permission from the inventor. Like copyrights, patents last for a limited period of time, after which the inventions covered by them enter the public domain and can be used by anyone.

No legal restriction on use

A creative work is said to be in the public domain if there are no laws which restrict its use by the public at large. For instance, a work may be in the public domain if no laws establish proprietary rights over the work, or if the work or its subject matter are specifically excluded from existing laws.

Because proprietary rights are founded in national laws, an item may be public domain in one jurisdiction but not another. For instance, some works of literature are public domain in the United States but not in the European Union and vice versa.

The underlying idea that is expressed or manifested in the creation of a work generally cannot be the subject of copyright law (see idea-expression divide). Mathematical formulæ will therefore generally form part of the public domain, to the extent that their expression in the form of software is not covered by copyright; however, algorithms can be the subject of a software patent in some jurisdictions.