Public Domain, sometimes called "royalty-free" or "copyright-free", or just "free', describes materials that are not under copyright protection which may be freely copied and used.
Examples include the Bible, Shakespeare, Government publications, and even Mark Twain.
Public Domain materials may be free because their copyright protection has expired or because they never had copyright protection. For instance, their creator may have made them freely available.
Most works published before 1923 in the United States are now in the Public Domain.