Internationally, the public domain is the body of creative works and other knowledge—writing, artwork, music, science, inventions, and others—in which no person or organization has any proprietary interest (typically a government-granted monopoly such as a copyright or patent). Such works and inventions are considered part of the public's cultural heritage, and anyone can use and build upon them without restriction (not taking into account laws concerning safety, export, etc).
While copyright was created to protect the financial incentive of those doing creative work as a means to encourage more creative work, works in the public domain just exist as such, and the public have the right to use and reuse the creative work of others without financial or social burden.
Without some kind of grant of monopoly rights—so-called "intellectual property rights"—all works belong to the public domain. When copyright or other protections reach the end of their life, works are said to revert to the public domain.
The abbreviation of public domain is PD.
Absence of legal protection
Creative works are in the public domain wherever no law exists to establish proprietary rights, or where the subject matter is specifically excluded from existing laws. For example, most mathematical formulas are not subject to copyrights or patents in most of the world (although their application in the form of computer programs can be patented). Likewise, works that were created long before such laws were passed are part of the public domain, such as the works of William Shakespeare and Ludwig van Beethoven and the inventions of Archimedes. Also, works of the United States Government are excluded from copyright law.
Most copyrights and patents have a finite term; when this expires, the work or invention falls into the public domain. In most of the world, patents expire 20 years after they are filed. Trademarks expire soon after the mark becomes a generic term. Copyrights are more complex; generally, they expire in all countries except Guatemala, Mexico, Samoa and Colombia when all of the following conditions are satisfied:
- The work was created and first published before January 1, 1923, or at least 95 years before January 1 of the current year, whichever is later.
- The last surviving author died at least 70 years before January 1 of the current year.
- No Berne Convention signatory has passed a perpetual copyright on the work.
- Neither the United States nor the European Union has passed a copyright term extension since these conditions were last updated. (This must be a condition because the exact numbers in the other conditions depend on the state of the law at any given moment.)
These conditions are based on the intersection of United States and European Union copyright law, which most other Berne Convention signatories recognize. Note that copyright term extension under U.S. tradition does not restore copyright to PD works (hence the 1923 date), but European tradition does because the EU harmonization was based on the copyright term in Germany, which had already been extended to life plus 70. Note further that works created by a United States government agency fall into PD at the moment of creation.
The situation with respect to British government works is a little more complex, but still relatively easy to understand. British government works are restricted by either Crown Copyright or Parliamentary Copyright. Published Crown Copyright works become public domain at the end of the year 50 years after they were published, unless the author of the work held copyright and assigned it to the Crown. In that case, the copyright term is the usual life of author plus 70 years. Unpublished Crown Copyright documents become public domain at the end of the year 125 years after they were first created. However, under the legislation that created this rule, and abolished the traditional common law perpetual copyright protection of unpublished works, no unpublished works will become public domain until 50 years after the legislation came into effect. Since the legislation became law on 1 August 1989, no unpublished works will become public domain under this provision until 2039. Parliamentary Copyright documents become public domain at the end of the year 50 years after they were published. Crown Copyright is waived on some government works provided that certain conditions are met.
These numbers reflect the most recent extensions of copyright in the United States and Europe. Canada and Australia have not as of 2004 passed similar twenty-year extensions. Consequently, their copyright expiry times are still life of the author plus 50 years. As a result, characters such as Mickey Mouse, and works ranging from Peter Pan to the stories of H. P. Lovecraft are public domain in both places. (The copyright status of Lovecraft's work is debatable, as no copyright renewals, which were necessary under the laws of that time, have been found. Also, two competing parties have independently claimed copyright ownership on his work.)
As with most other British Commonwealth countries, Canada and Australia follow the general lead of the United Kingdom on copyright of government works. Both have a version of Crown Copyright which lasts for 50 years from publication. New Zealand also has Crown Copyright protection, but has a much greater time length of protection at 100 years from the date of publication. Ireland also has a fifty year term on government works, although since it is no longer a monarchy, such protection is, of course, not called Crown Copyright. India has a government copyright of sixty years from publication, to coincide with its somewhat unusual life of the author plus sixty years term of copyright.
Examples of inventions whose patents have expired include the inventions of Thomas Edison. Examples of works whose copyrights have expired include the works of Carlo Collodi and most of the works of Mark Twain. Examples of works under a statutory perpetual copyright include many of the Peter Pan works by J. M. Barrie; this was granted by the British government and applies only within the United Kingdom. Other works, such as the works of The Walt Disney Company are not under a de jure statutory perpetual copyright because the United States Constitution requires copyrights to last "for limited Times" (Article I, section 8, clause 8). However, the limits have been retroactively extended several times, leading to longer and longer protections. Critics have observed that the extensions have taken place right before noteworthy works from Disney and others were about to expire, concluding that such copyright term extensions add up to de facto perpetual copyright. Disney and other large publishers routinely provide millions of U.S. dollars in campaign money to legislators, allegedly in exchange for these continued extensions.
Disclaimer of interest
In the past, in some jurisdictions such as the USA, a work would enter the public domain with respect to copyright if it was released without a copyright notice. This is no longer the case. Any work receives copyright by default and copyright law generally doesn't provide any special means to "abandon" copyright so that a work can enter the public domain (in the USA, the Computer Software Rental Amendments Act of 1990 provides a registration mechanism for public domain computer programs at the Library of Congress, but it is still not explained how the work should be placed in the public domain in the first place).
A copyright holder can explicitly disclaim any proprietary interest in the work, effectively granting it to the public domain, by providing a licence to this effect. A suitable licence will grant permission for all of the acts which are restricted by copyright law.
With regards to patents on the other hand, publishing the details of an invention before applying for a patent will generally place an invention in the public domain and prevent its subsequent patenting by others. For example, once a journal publishes a mathematical formula, it may no longer be used as the core of a claim in a software patent. There is an exception to this, however: in US (not European) law, an inventor may file a patent claim up to one year after publishing it (but not, of course, if someone else published it first).
Laws may make some types of works and inventions ineligible for monopoly; such works immediately enter the public domain upon publication. For example, US copyright law, 17 U.S.C. § 105, releases all works created by the US government into the public domain, patent applications as part of the terms of granting the patent to the invention are public domain, patent law excludes inventions that obviously follow from prior art, and agreements that Germany signed at the end of World War I released such trademarks as "aspirin" and "heroin" into the public domain in many areas.
Note that there are many works that are not part of the public domain, but for which the owner of some proprietary rights has chosen not to enforce those rights, or to grant some subset of those rights to the public. See, for example, the Free Software Foundation which creates copyrighted software and licenses it without charge to the public for most uses under a class of license called "copyleft", forbidding only proprietary redistribution. See also Wikimd, which does much the same thing with its content under the GNU Free Documentation License. Sometimes such work is mistakenly referred to as "public domain" in colloquial speech.
Note also that while some works (especially musical works) may be in the public domain, U.S. law considers transcriptions or performances of those works to be derivative works, potentially subject to their own copyrights.
The role in the society
"Public access to literature, art, music, and film is esssential to preserving and building on our cultural heritage. Many of the most important works of American culture have drawn upon the creative potential of the public domain. Frank Capra's It's a Wonderful Life is a classic example of a film that did not enjoy popular success until it entered the public domain. Other icons such as Snow White, Pinocchio, Santa Claus, and Uncle Sam grew out of public domain figures." ()
- Chris Sprigman's article THE MOUSE THAT ATE THE PUBLIC DOMAIN: Disney, The Copyright Term Extension Act, And Eldred v. Ashcroft
- Copyright Research and Information center - about the copyright law in Japan
- When U.S. works pass into the public domain.
- Short list of uncopyrightable things in the US
-  for a summary list of copyright terms in other countries.
- MPEG video recordings of panel discussions from the Conference on the Public Domain (2001) panelists include Eben Moglen, Robin Gross, and Lawrence Lessig