The public domain is an intellectual property designation for the range of content that is not owned or controlled by anyone. These materials are "public property", and available for anyone to use freely 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.