Public Domain

Any source code, program or other intellectual work that has no specific owner and can therefore be used in any way you like. An intellectual work can enter the public domain through two main mechanisms:
 * The author of the work specifically places his work in the public domain, usually "for the greater good" or out of complete indifference.
 * The copyright runs out according to local copyright laws, the terms for these vary from country to country and can vary with types of works.
 * Intellectual works, for instance books, source code, a song or a picture of any sort, usually run out from a set time after the death of the main creator of the work or the death or the last main creator of the work. This is typically 50 to 75 years after the death, but varies from country to country.
 * Mechanical recordings, like sound recordings, run out after a certain time period since its creation, typically 25 to 50 years, but also varies from country to country.

Note that the author of the work has to specifically place his work in the public domain, since the Berne Convention of 1886 the granting of copyright has been automatic, i.e. you do not need to copyright a work to protect it, it is automatically copyrighted as soon as you create it.

Also note that a work can be a compound work, i.e. both an intellectual work and a mechanical recording, for instance a movie or a music recording, and the definition can vary from country to country. In the case of a sound recording for instance you may use it as you wish after the copyright runs out, however the intellectual work used in the creation of it can still be copyrighted, so you may have to pay the writer of the song, the lyrics and the arranger mechanical royalties, even though you no longer have to pay for the non-intellectual work like band, vocalist, recordist etc. In the case of a movie this can be both ways, some countries treat movies as intellectual works because cutting and direction are considered intellectual pursuits, while others consider a movie a mechanical recording as cutting and direction etc. are seen as trades.

There are a few ways that a work that is otherwise believed to be copyrighted can fall into the public domain: It is not possible to claim ownership of a public domain work except by adding to it, you cannot grab a source code that is PD and claim ownership of it and add a license to it even if that is actually common practice in the open source world, however if you base your work on a PD work your additions become your copyright and as the work is a compound the whole work becomes copyrighted. That does however not change the status of the original work in any way, and the changes to the original work have to be non-trivial.
 * In some countries, notably the USA, all work that is paid for by the public purse has by law to fall into the public domain. There are ways around this for the creators of the intellectual work, but there are some famous cases of software that one person or organisation has claimed ownership and copyright of have later been found to be in the public domain by a court of law. A couple of notable examples of this are UCSD Pascal and the Falcon database (basis of dBase II).
 * Work created in one of the old "Pirate states" (Brazil, USA, Israel, Taiwan, et al. countries that had laws that favoured local ownership of foreign intellectual property) can be exempt if it was created before the country in question joined the Berne convention, quite a few of them joined after the Paris Convention of 1971 and therefore work done before that can in some cases become PD. The US did not join until 1988 but had adopted legislation from the convention piecemeal over time since 1910 so it had effectively "joined" in the 1970's and the cutoff date is often seen as 1972 over there, Brazil did not join until 1975 but did not really start to enforce it until the 1990's, but work created in Brazil before 75 can be PD. Israel had always been a signatory to the Berne convention but did not implement "moral rights" until the 1980's which gave local publishers a loophole to pirate works, Taiwan has never joined the Berne Convention so Taiwanese copyright is controlled by individual treaties and as Taiwan has only signed up with Spain, Hong Kong, USA and the UK, you are free to use Taiwanese IP in the rest of the world without issue.
 * Work from some countries can become PD either globally or in certain countries if an international arbitration court finds one country is not playing ball in terms of intellectual agreements. This is typically a specific type of work and sometimes within a specific time-frame. European broadcasters no longer have to pay performance royalties to USA recording artist and a few smaller nations that have won cases against the USA at the IPO that the USA government has refused to comply with, and as the countries have been too small to have any chance of enforcing the judgements on their own the IPO has granted them the right to treat all USA IP as PD, albeit on a local basis only.

Links

 * http://www.wipo.int/treaties/en/text.jsp?file_id=283698 Berne convention.