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Public Domain Mark (PDM) is a symbol used to indicate that a work is free of known copyright restrictions and therefore in the public domain. It is analogous to the copyright symbol, which is commonly used to indicate as copyright notice that a work is copyrighted. The Public Domain Mark was developed by Creative Commons.
Uses
For instance Wikimedia Commons has on February 2016 2.9 million works (~10% of all works) listed as PDM.
References
Public Domain Mark Wikipedia(Text) CC BY-SA