Any intellectual property which is not owned or controlled by anyone is a work in public domain.  These materials are public property and are available for free use.  Although, the public domain can be defined in connection to several forms of intellectual property, it is most often discussed in contrast to works restricted by copyright.  All creative works, which are not protected under copyright law are said to be in the public domain.

Copyright gives the owner of an original work exclusive right for a certain time period in relation to that work.  Copyright protected rights include rights to publication, distribution and adaptation.  Once the term of the copyright ends, the work enters the public domain.  It is estimated that currently, of all the published books, only 15% are in the public domain.  The rest are still under copyright protection.  The law regarding the works in the public domain is a matter of common law rather than a statute.

Part of the public domain consists of works that are excluded from federal copyright protection.  The clearest example is that of US Government works, which are precluded from copyright.  Other examples are state judicial opinions, legislative enactments, and other official documents.