When a work is in the public domain, it is free for use by anyone for any purpose. Public domain is the purest form of open/free, since no one owns owns or controls the material in any way. In other words, it is free of any license. For more information, see Wikipedia's article on public domain.
When does a work enter the public domain?
Cornell University has also provided a handy table about copyright term and the public domain in the United States.
Creative Commons public domain tools
- CC0 (occasionally written as CC Zero) is a waiver that allows copyright holders to place works in the public domain to the extent legally possible, worldwide.
- CC's public domain certification serves to mark works that are in the public domain already. A next-generation version of this tool, to be called a "public domain assertion tool" is under development.
See http://creativecommons.org/publicdomain for more information on CC's public domain tools.
Public domain content in practice is any content without copyright. It may be deliberately freed from restrictions by the copyright owner, or the copyright may lapse after a certain time. All content created by US government employees as part of their employment is public domain, making US federal sites (considered together) probably the largest source of public domain content by far.
The legal details in practice vary between countries. More information about public domain can be found at Wikipedia's article on the public domain.
Appropedia's Public Domain Search
Appropedia's Public Domain Search was started in late 2007 when it was discovered there was no effective public domain search available. This operates on a completely different method to searches using the Creative Commons marks. It uses a manually maintained index of sites known to be public domain - thus it is does not yield 100% public domain results, and content must be checked to confirm public domain status. Reliability is expected to improve, and feedback by users is encouraged.
As CC public domain tools become widely used, searches based on the CC marks would be expected to take over from Appropedia's Public Domain Search.