Content to which copyright does not apply, for example because it has expired or it never existed, is free for any kind of use by anyone and is said to be in the public domain. CC0, one of the licences of Creative Commons, is a ‘public domain dedication’ which attempts so far as possible to renounce all rights in the work and place it in the public domain. Source: ODH
The Public Domain means data is free for use by anyone for any purpose without restriction under copyright law. Public domain is the purest form of open/free, since no one owns or controls the material in any way. For the official documents in most of the European countries it is the most convenient licence.
Source: Creative Commons.